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/sef_init.c

@@ -2,6 +2,7 @@
 #include <assert.h>
 #include <unistd.h>
 #include <minix/sysutil.h>
+#include <string.h>
 
 /* SEF Init callbacks. */
 static struct sef_cbs {
@@ -62,6 +63,7 @@ static int process_init(int type, sef_init_info_t *info)
       break;
   }
 
+  memset(&m, 0, sizeof(m));
   m.m_source = sef_self_endpoint;
   m.m_type = RS_INIT;
   m.RS_INIT_RESULT = result;