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/libi2cdriver/i2cdriver.c

@@ -26,7 +26,7 @@ i2cdriver_announce(uint32_t bus)
 	}
 
 	/* Publish a driver up event. */
-	r = ds_retrieve_label_name(label, getprocnr());
+	r = ds_retrieve_label_name(label, sef_self());
 	if (r != OK) {
 		panic("unable to get own label: %d\n", r);
 	}