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

drivers/vnd/vnd.c

@@ -384,9 +384,10 @@ vnd_ioctl(devminor_t UNUSED(minor), unsigned long request, endpoint_t endpt,
 		 * making the IOCTL call.  The result is either a newly
 		 * allocated file descriptor or an error.
 		 */
-		if ((state.fd = copyfd(user_endpt, vnd.vnd_fildes,
-		    COPYFD_FROM)) == -1)
-			return -errno;
+		if ((r = copyfd(user_endpt, vnd.vnd_fildes, COPYFD_FROM)) < 0)
+			return r;
+
+		state.fd = r;
 
 		/* The target file must be regular. */
 		if (fstat(state.fd, &st) == -1) {