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

servers/is/main.c

@@ -16,8 +16,6 @@ static message m_out;		/* the output message used for reply */
 static endpoint_t who_e;	/* caller's proc number */
 static int callnr;		/* system call number */
 
-extern int errno;	/* error number set by system library */
-
 /* Declare some local functions. */
 static void get_work(void);
 static void reply(int whom, int result);