Introduce set_force_quit_flag and change type of sync_quit_force_run

Introduce set_force_quit_flag and change type of sync_quit_force_run

At the moment, handle_sigterm() in event-top.c does the following:

  sync_quit_force_run = 1;
  set_quit_flag ();

This was used several more times in a later patch in this series, so
I'm introducing (at Pedro's suggestion) a new function named
'set_force_quit_flag'.  It simply sets sync_quit_force_run and also
calls set_quit_flag().  I've revised the later patch to call
set_force_quit_flag instead.

I noticed that sync_quit_force_run is declared as an int but is being
used as a bool, so I also changed its type to bool in this commit.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26761
Approved-By: Pedro Alves <pedro@palves.net>