The why3server supports the "interrupt" command which removes a task.
But until now, the server didn't actually kill any already running
processes. This is now implemented.

Background is that I plan some refactorings which may make use of the
"interrupt" command, so I want it to work properly.

This new code is untested on windows, because no client code actually
uses the interrupt command in SPARK. But I tested that it doesn't cause
regressions. It will be later tested by the new code I intend to write.

*server-unix.c
*server-win.c
(overall): outsource some little code to new proc unit
(handle_msg): actually kill processes

* proc.c - new file
(os_kill): new platform-dependent function to kill a process by pid
(init_process_list): new function to init the process list
(kill_processes): new function to kill processes from process list with
  a certain key
(free_process): release memory held by a process record

Change-Id: I7238578061c65883423f8ec538d79d9de3fb717b
(cherry picked from commit b17873a181ff56abda9d09bc8d10c29ba036c019)