Unmask SIGTERM and enable PIDFile for systemd

Unmask SIGTERM and enable PIDFile for systemd

systemd service stop logic uses SIGTERM to stop services. However,
system_server daemon ignores that signal:

   systemd[1]: Stopping Start the system server service...
   system_server[3392]: main.c: sig_quit(47) > received SIGTERM signal 15

Due to TimeoutStopSec=2 being set, systemd waits 2 seconds until it
proceeds with kill:

   systemd[1]: system-server.service stopping timed out. Killing.
   systemd[1]: system-server.service: main process exited, code=killed,
               status=9/KILL

This delays system shutdown time.

Additionally, add PIDFile to tell systemd the .pid system_server uses.
This is useful since system-service.service uses 'forking' and is run
via device-daemon script.

Bug-Tizen: TC-1502

Change-Id: I5b00cf11a67ab15b651d754d270b0549c3f806f4
Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>