SYSENTER/SYSCALL support

. add cpufeature detection of both
	. use it for both ipc and kernelcall traps, using a register
	  for call number
	. SYSENTER/SYSCALL does not save any context, therefore userland
	  has to save it
	. to accomodate multiple kernel entry/exit types, the entry
	  type is recorded in the process struct. hitherto all types
	  were interrupt (soft int, exception, hard int); now SYSENTER/SYSCALL
	  is new, with the difference that context is not fully restored
	  from proc struct when running the process again. this can't be
	  done as some information is missing.
	. complication: cases in which the kernel has to fully change
	  process context (i.e. sigreturn). in that case the exit type
	  is changed from SYSENTER/SYSEXIT to soft-int (i.e. iret) and
	  context is fully restored from the proc struct. this does mean
	  the PC and SP must change, as the sysenter/sysexit userland code
	  will otherwise try to restore its own context. this is true in the
	  sigreturn case.
	. override all usage by setting libc_ipc=1

lib/libsys/kernel_call.c

@@ -4,6 +4,6 @@
 int _kernel_call(int syscallnr, message *msgptr)
 {
   msgptr->m_type = syscallnr;
-  _do_kernel_call(msgptr);
+  do_kernel_call(msgptr);
   return(msgptr->m_type);
 }

lib/libsys/taskcall.c

@@ -14,7 +14,7 @@ register message *msgptr;
   int status;
 
   msgptr->m_type = syscallnr;
-  status = _sendrec(who, msgptr);
+  status = sendrec(who, msgptr);
   if (status != 0) return(status);
   return(msgptr->m_type);
 }