Server/driver protocols: no longer allow third-party copies.

Before safecopies, the IO_ENDPT and DL_ENDPT message fields were needed
to know which actual process to copy data from/to, as that process may
not always be the caller. Now that we have full safecopy support, these
fields have become useless for that purpose: the owner of the grant is
*always* the caller. Allowing the caller to supply another endpoint is
in fact dangerous, because the callee may then end up using a grant
from a third party. One could call this a variant of the confused
deputy problem.

From now on, safecopy calls should always use the caller's endpoint as
grant owner. This fully obsoletes the DL_ENDPT field in the
inet/ethernet protocol. IO_ENDPT has other uses besides identifying the
grant owner though. This patch renames IO_ENDPT to USER_ENDPT, not only
because that is a more fitting name (it should never be used for I/O
after all), but also in order to intentionally break any old system
source code outside the base system. If this patch breaks your code,
fixing it is fairly simple:

- DL_ENDPT should be replaced with m_source;
- IO_ENDPT should be replaced with m_source when used for safecopies;
- IO_ENDPT should be replaced with USER_ENDPT for any other use, e.g.
  when setting REP_ENDPT, matching requests in CANCEL calls, getting
  DEV_SELECT flags, and retrieving of the real user process's endpoint
  in DEV_OPEN.

The changes in this patch are binary backward compatible.

kernel/arch/i386/do_iopenable.c

@@ -2,7 +2,7 @@
  *   m_type:	SYS_IOPENABLE
  *
  * The parameters for this system call are:
- *    m2_i2:	IO_ENDPT	(process to give I/O Protection Level bits)
+ *    m2_i2:	IOP_ENDPT	(process to give I/O Protection Level bits)
  *
  * Author:
  *    Jorrit N. Herder <jnherder@cs.vu.nl>
@@ -22,9 +22,9 @@ PUBLIC int do_iopenable(struct proc * caller, message * m_ptr)
   int proc_nr;
 
 #if 1 /* ENABLE_USERPRIV && ENABLE_USERIOPL */
-  if (m_ptr->IO_ENDPT == SELF) {
+  if (m_ptr->IOP_ENDPT == SELF) {
 	proc_nr = _ENDPOINT_P(caller->p_endpoint);
-  } else if(!isokendpt(m_ptr->IO_ENDPT, &proc_nr))
+  } else if(!isokendpt(m_ptr->IOP_ENDPT, &proc_nr))
 	return(EINVAL);
   enable_iop(proc_addr(proc_nr));
   return(OK);