Merge of David's ptrace branch. Summary:

o Support for ptrace T_ATTACH/T_DETACH and T_SYSCALL
o PM signal handling logic should now work properly, even with debuggers
  being present
o Asynchronous PM/VFS protocol, full IPC support for senda(), and
  AMF_NOREPLY senda() flag

DETAILS

Process stop and delay call handling of PM:
o Added sys_runctl() kernel call with sys_stop() and sys_resume()
  aliases, for PM to stop and resume a process
o Added exception for sending/syscall-traced processes to sys_runctl(),
  and matching SIGKREADY pseudo-signal to PM
o Fixed PM signal logic to deal with requests from a process after
  stopping it (so-called "delay calls"), using the SIGKREADY facility
o Fixed various PM panics due to race conditions with delay calls versus
  VFS calls
o Removed special PRIO_STOP priority value
o Added SYS_LOCK RTS kernel flag, to stop an individual process from
  running while modifying its process structure

Signal and debugger handling in PM:
o Fixed debugger signals being dropped if a second signal arrives when
  the debugger has not retrieved the first one
o Fixed debugger signals being sent to the debugger more than once
o Fixed debugger signals unpausing process in VFS; removed PM_UNPAUSE_TR
  protocol message
o Detached debugger signals from general signal logic and from being
  blocked on VFS calls, meaning that even VFS can now be traced
o Fixed debugger being unable to receive more than one pending signal in
  one process stop
o Fixed signal delivery being delayed needlessly when multiple signals
  are pending
o Fixed wait test for tracer, which was returning for children that were
  not waited for
o Removed second parallel pending call from PM to VFS for any process
o Fixed process becoming runnable between exec() and debugger trap
o Added support for notifying the debugger before the parent when a
  debugged child exits
o Fixed debugger death causing child to remain stopped forever
o Fixed consistently incorrect use of _NSIG

Extensions to ptrace():
o Added T_ATTACH and T_DETACH ptrace request, to attach and detach a
  debugger to and from a process
o Added T_SYSCALL ptrace request, to trace system calls
o Added T_SETOPT ptrace request, to set trace options
o Added TO_TRACEFORK trace option, to attach automatically to children
  of a traced process
o Added TO_ALTEXEC trace option, to send SIGSTOP instead of SIGTRAP upon
  a successful exec() of the tracee
o Extended T_GETUSER ptrace support to allow retrieving a process's priv
  structure
o Removed T_STOP ptrace request again, as it does not help implementing
  debuggers properly
o Added MINIX3-specific ptrace test (test42)
o Added proper manual page for ptrace(2)

Asynchronous PM/VFS interface:
o Fixed asynchronous messages not being checked when receive() is called
  with an endpoint other than ANY
o Added AMF_NOREPLY senda() flag, preventing such messages from
  satisfying the receive part of a sendrec()
o Added asynsend3() that takes optional flags; asynsend() is now a
  #define passing in 0 as third parameter
o Made PM/VFS protocol asynchronous; reintroduced tell_fs()
o Made PM_BASE request/reply number range unique
o Hacked in a horrible temporary workaround into RS to deal with newly
  revealed RS-PM-VFS race condition triangle until VFS is asynchronous

System signal handling:
o Fixed shutdown logic of device drivers; removed old SIGKSTOP signal
o Removed is-superuser check from PM's do_procstat() (aka getsigset())
o Added sigset macros to allow system processes to deal with the full
  signal set, rather than just the POSIX subset

Miscellaneous PM fixes:
o Split do_getset into do_get and do_set, merging common code and making
  structure clearer
o Fixed setpriority() being able to put to sleep processes using an
  invalid parameter, or revive zombie processes
o Made find_proc() global; removed obsolete proc_from_pid()
o Cleanup here and there

Also included:
o Fixed false-positive boot order kernel warning
o Removed last traces of old NOTIFY_FROM code

THINGS OF POSSIBLE INTEREST

o It should now be possible to run PM at any priority, even lower than
  user processes
o No assumptions are made about communication speed between PM and VFS,
  although communication must be FIFO
o A debugger will now receive incoming debuggee signals at kill time
  only; the process may not yet be fully stopped
o A first step has been made towards making the SYSTEM task preemptible

drivers/printer/printer.c

@@ -90,7 +90,7 @@ PRIVATE int revive_pending;	/* set to true if revive is pending */
 PRIVATE int revive_status;	/* revive status */
 PRIVATE int done_status;	/* status of last output completion */
 PRIVATE int oleft;		/* bytes of output left in obuf */
-PRIVATE char obuf[128];		/* output buffer */
+PRIVATE unsigned char obuf[128];	/* output buffer */
 PRIVATE unsigned char *optr;		/* ptr to next char in obuf to print */
 PRIVATE int orig_count;		/* original byte count */
 PRIVATE int port_base;		/* I/O port for printer */
@@ -113,7 +113,7 @@ FORWARD _PROTOTYPE( void prepare_output, (void) );
 FORWARD _PROTOTYPE( void do_initialize, (void) );
 FORWARD _PROTOTYPE( void reply, (int code,int replyee,int proc,int status));
 FORWARD _PROTOTYPE( void do_printer_output, (void) );
-FORWARD _PROTOTYPE( void do_signal, (message *m_ptr) );
+FORWARD _PROTOTYPE( void do_signal, (void) );
 
 
 /*===========================================================================*
@@ -140,13 +140,11 @@ PUBLIC void main(void)
 			case HARDWARE:
 				do_printer_output();
 				break;
-			case SYSTEM:
-				do_signal(&pr_mess);
-				break;
 			case RS_PROC_NR:
 				notify(pr_mess.m_source);
 				break;
 			case PM_PROC_NR:
+				do_signal();
 				break;
 			default:
 				reply(TASK_REPLY, pr_mess.m_source,
@@ -175,11 +173,11 @@ PUBLIC void main(void)
 /*===========================================================================*
  *				 do_signal	                             *
  *===========================================================================*/
-PRIVATE void do_signal(m_ptr)
-message *m_ptr;					/* signal message */
+PRIVATE void do_signal()
 {
-  int sig;
-  sigset_t sigset = m_ptr->NOTIFY_ARG;
+  sigset_t sigset;
+
+  if (getsigset(&sigset) != 0) return;
   
   /* Expect a SIGTERM signal when this server must shutdown. */
   if (sigismember(&sigset, SIGTERM)) {