gdb/remote-fileio.c: Eliminate custom SIGINT signal handler

gdb/remote-fileio.c: Eliminate custom SIGINT signal handler

... and fix Ctrl-C races.

The current remote-fileio.c SIGINT/EINTR code can lose Ctrl-C --
there's a period where SIG_IGN is installed as signal handler, for
example.

Since:

 - remote.c no longer installs a custom SIGINT handler;

 - The current remote-fileio.c SIGINT handler is basically the same as
   the default SIGINT handler (event-top.c:handle_sigint), in
   principle, except that instead of setting the quit flag, it sets a
   separate flag.

I think we should be able to completely remove the remote-fileio.c
SIGINT handler, and centralize on the quit flag, thus fixing the
Ctrl-C race.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* remote-fileio.c (remote_fio_ctrl_c_flag, remote_fio_sa)
	(remote_fio_osa)
	(remote_fio_ofunc, remote_fileio_sig_init, remote_fileio_sig_set)
	(remote_fileio_sig_exit, remote_fileio_ctrl_c_signal_handler):
	Delete.
	(remote_fileio_o_quit_handler): New global.
	(remote_fileio_quit_handler): New function.
	(remote_fileio_reply): Check the quit flag instead of the custom
	'remote_fio_ctrl_c_flag' flag.  Restore the quit handler instead
	of changing the SIGINT handler.
	(do_remote_fileio_request): Override the quit handler instead of
	changing the SIGINT handler.