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/ircterm.h

@@ -709,7 +709,7 @@ extern char default_pair[2];
 #endif
 #endif
 
-	RETSIGTYPE 	term_cont 		(int);
+SIGNAL_HANDLER(term_cont);
 	void 		term_beep 		(void);
 	int		term_echo 		(int);
 	int		term_init 		(char *);