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/taskcall.c

@@ -6,13 +6,6 @@
 #include <lib.h>
 #include <minix/syslib.h>
 
-int _sendcall(endpoint_t who, int type, message *msgptr)
-{
-  msgptr->m_type = type;
-  return send(who, msgptr);
-}
-
-
 int _taskcall(who, syscallnr, msgptr)
 endpoint_t who;
 int syscallnr;