Modularise handling of DCC offers

There's several different types of DCC offers, all of which need slightly
different handling.  Previously they were all handled by the monster function
register_dcc_type() - this breaks them out into seperate functions for handling
SEND, RESEND, CHAT and BOT offers, moves common code into static helper
functions and renames the entry point from ctcp.c to handle_dcc_offer().

This will allow adding a way for modules to register DCC offer types that
they're interested in.

This also moves rename_file() from misc.c into dcc.c, where its only user is.

include/misc.h

@@ -81,7 +81,6 @@ extern LastMsg last_sent_ctcp[2];
 	char	*BX_random_str (int, int);
 	void	check_auto_away (time_t);
 ChannelList *	BX_prepare_command (int *, char *, int);
-	int	rename_file (char *, char **);
 	void	putlog (int, ...);
 
 	void	add_mode_buffer ( char *, int);