Change send_msg_to_channels() to remove 'channels' argument and always use PRIVMSG

The 'channels' argument was unnecessary because the channel list passed to it would
always be from get_server_channels(server) anyway.

Changing the message argument from a protocol message format string to a plain payload
string for PRIVMSG means printf-escaping of the argument is avoided in the caller.
This simplifies the callers and means we can remove the last use of quote_it().

This change also switches set_server_away() to use send_msg_to_channels() instead of
open-coding the equivalent, which simplifies that function a lot.

We are changing the signature of a function exported to modules here; however none
of the in-tree modules use this function so it should be OK.

include/server.h

@@ -375,7 +375,7 @@ struct sockaddr_foobar	get_server_local_addr		(int);
 struct sockaddr_foobar	get_server_uh_addr		(int);
 NotifyItem	*get_server_notify_list		(int);
 	void	BX_send_msg_to_nicks		(ChannelList *, int, char *);        
-	void	BX_send_msg_to_channels		(ChannelList *, int, char *);        
+	void	BX_send_msg_to_channels		(int, const char *);        
 	int	BX_is_server_queue			(void);
 	int	save_servers			(FILE *);	        
 	void	add_split_server		(char *, char *, int);