libsys: various updates

- move system calls for use by services from libminlib into libsys;
- move srv_fork(2) and srv_kill(2) from RS and into libsys;
- replace getprocnr(2) with sef_self(3);
- rename previous getnprocnr(2) to getprocnr(2);
- clean up getepinfo(2);
- change all libsys calls that used _syscall to use _taskcall, so as
  to avoid going through errno to pass errors; this is already how
  most calls work anyway, and many of the calls previously using
  _syscall were already assumed to return the actual error;
- initialize request messages to zero, for future compatibility
  (note that this does not include PCI calls, which are in need of a
  much bigger overhaul, nor kernel calls);
- clean up more of dead DS code as a side effect.

Change-Id: I8788f54c68598fcf58e23486e270c2d749780ebb

lib/libsys/sched_stop.c

@@ -1,5 +1,6 @@
 #include "syslib.h"
 #include <assert.h>
+#include <string.h>
 
 /*===========================================================================*
  *				sched_stop				     *
@@ -18,6 +19,7 @@ int sched_stop(endpoint_t scheduler_e, endpoint_t schedulee_e)
 	assert(_ENDPOINT_P(scheduler_e) >= 0);
 	assert(_ENDPOINT_P(schedulee_e) >= 0);
 
+	memset(&m, 0, sizeof(m));
 	m.SCHEDULING_ENDPOINT	= schedulee_e;
 	if ((rv = _taskcall(scheduler_e, SCHEDULING_STOP, &m))) {
 		return rv;