Consistently use SIGNAL_HANDLER() macro to define signal handler functions

With this change, the signal handler signature is defined in only one place.

Also make most signal handler functions static (those that are only referred to in irc.c).

include/screen.h

@@ -40,7 +40,6 @@
 	void	window_redirect (char *, int);
 	void	redraw_resized (Window *, ShrinkInfo, int);
 	void	close_all_screen (void);
-RETSIGTYPE	sig_refresh_screen (int);
 	int	check_screen_redirect (char *);
 	void	BX_kill_screen (Screen *);
 	ShrinkInfo	resize_display (Window *);