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/vm_fork.c

@@ -1,6 +1,7 @@
 
 #include "syslib.h"
 
+#include <string.h>
 #include <minix/vm.h>
 
 /*===========================================================================*
@@ -11,6 +12,7 @@ int vm_fork(endpoint_t ep, int slot, endpoint_t *childep)
     message m;
     int result;
 
+    memset(&m, 0, sizeof(m));
     m.VMF_ENDPOINT = ep;
     m.VMF_SLOTNO = slot;