Fix signal handler/event-loop races

GDB's core signal handling suffers from a classical signal handler /
mainline code race:

  int
  gdb_do_one_event (void)
  {
  ...
  /* First let's see if there are any asynchronous signal handlers
       that are ready.  These would be the result of invoking any of the
       signal handlers.  */
    if (invoke_async_signal_handlers ())
      return 1;
  ...
    /* Block waiting for a new event.  (...).  */

    if (gdb_wait_for_event (1) < 0)
      return -1;
  ...
  }

If a signal is delivered while gdb is blocked in the poll/select
inside gdb_wait_for_event, then the select/poll breaks with EINTR,
we'll loop back around and call invoke_async_signal_handlers.

However, if the signal handler runs between
invoke_async_signal_handlers and gdb_wait_for_event,
gdb_wait_for_event will block, until the next unrelated event...

The fix is to a struct serial_event, and register it in the set of
files that select/poll in gdb_wait_for_event waits on.  The signal
handlers that defer work to invoke_async_signal_handlers call
mark_async_signal_handler, which is adjusted to also set the new
serial event in addition to setting a flag, and is thus now is
garanteed to immediately unblock the next gdb_select/poll call, up
until invoke_async_signal_handlers is called and the event is cleared.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* event-loop.c: Include "ser-event.h".
	(async_signal_handlers_serial_event): New global.
	(async_signals_handler, initialize_async_signal_handlers): New
	functions.
	(mark_async_signal_handler): Set
	async_signal_handlers_serial_event.
	(invoke_async_signal_handlers): Clear
	async_signal_handlers_serial_event.
	* event-top.c (async_init_signals): Call
	initialize_async_signal_handlers.

gdb/event-top.c

@@ -748,6 +748,8 @@ gdb_readline_no_editing_callback (gdb_client_data client_data)
 void
 async_init_signals (void)
 {
+  initialize_async_signal_handlers ();
+
   signal (SIGINT, handle_sigint);
   sigint_token =
     create_async_signal_handler (async_request_quit, NULL);