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

test/ds/README

@@ -10,8 +10,8 @@ How the test works
 ==================
 
 `dstest` tests the new DS API (excluding ds_subscribe() and ds_check()).
-test_u32, test_str, test_mem, test_label, and test_map test U32, STR, MEM, LABEL
-and MAP type respectively.
+test_u32, test_str, test_mem, and test_label test U32, STR, MEM, and LABEL
+type respectively.
 
 Invalid invokation is tested as well. Erroneous conditions are tested only once.
 For example, publishing an entry with same label name, but without