Make sure that the target user is able to join a local channel

Implement ERR_USERNOTONSERV(504) numeric and make sure that the
target user is on the same server when inviting other users to
local ("&") channels.

ircd-ratbox uses the ERR_USERNOTONSERV(504) numeric for this, and I
think this is a good idea -- other IRC daemons (like ircu) silently
drop such impossible invites, but thats not a big benefit ...

Idea by Cahata, thanks! Closes #183.

src/ngircd/irc-op.c

@@ -150,6 +150,14 @@ IRC_INVITE(CLIENT *Client, REQUEST *Req)
 		return IRC_WriteErrClient(from, ERR_NOSUCHNICK_MSG,
 					  Client_ID(Client), Req->argv[0]);
 
+	if (Req->argv[1][0] == '&') {
+		/* Local channel. Make sure the target user is on this server! */
+		if (Client_Conn(target) == NONE)
+			return IRC_WriteErrClient(from, ERR_USERNOTONSERV_MSG,
+						  Client_ID(Client),
+						  Req->argv[0]);
+	}
+
 	chan = Channel_Search(Req->argv[1]);
 	if (chan) {
 		/* Channel exists. Is the user a valid member of the channel? */

src/ngircd/messages.h

@@ -156,6 +156,7 @@
 #define ERR_UMODEUNKNOWNFLAG_MSG	"501 %s :Unknown mode"
 #define ERR_UMODEUNKNOWNFLAG2_MSG	"501 %s :Unknown mode \"%c%c\""
 #define ERR_USERSDONTMATCH_MSG		"502 %s :Can't set/get mode for other users"
+#define ERR_USERNOTONSERV_MSG		"504 %s %s :User is not on this server"
 #define ERR_NOINVITE_MSG		"518 %s :Cannot invite to %s (+V)"
 
 #ifdef ZLIB