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/libminlib/Makefile

@@ -30,11 +30,6 @@ SRCS+= servxcheck.c
 # queryparam
 SRCS+= paramvalue.c
 
-# Minix servers/drivers syscall. FIXME: these should be moved into libsys.
-SRCS+= 	checkperms.c copyfd.c getngid.c getnpid.c getnprocnr.c getnucred.c \
-	getnuid.c getprocnr.c mapdriver.c vm_memctl.c vm_set_priv.c \
-	vm_query_exit.c vm_update.c
-
 SRCS+= oneC_sum.c
 
 SUBDIR+= pkgconfig