bpo-37031: Reuse _PyRuntime.main_thread in signalmodule.c (GH-13538)

Remove main_thread and main_interp variables from signalmodule.c:
reuse _PyRuntime which already track the main thread and the main
interpreter.

* Remove #include <sys/types.h> which became useless: getpid() call
  has been removed.
* Add runtime argument to is_main()
* is_main() now gets the interpreter from runtime.