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/mapdriver.c

@@ -0,0 +1,17 @@
+#include "syslib.h"
+
+#include <string.h>
+#include <unistd.h>
+
+int
+mapdriver(char *label, devmajor_t major)
+{
+	message m;
+
+	memset(&m, 0, sizeof(m));
+	m.VFS_MAPDRIVER_LABEL = label;
+	m.VFS_MAPDRIVER_LABELLEN = strlen(label) + 1;
+	m.VFS_MAPDRIVER_MAJOR = major;
+
+	return _taskcall(VFS_PROC_NR, MAPDRIVER, &m);
+}