Allow channel admins to "de-admin" channel members

This patch fixes unsetting of channel user mode "+a" (channel admin)
and adds a better error message: without this patch, a channel admin is
unable to unset this mode.

This closes bug #142.

src/ngircd/irc-mode.c

@@ -697,9 +697,9 @@ Channel_Mode(CLIENT *Client, REQUEST *Req, CLIENT *Origin, CHANNEL *Channel)
 		/* --- Channel user modes --- */
 		case 'q': /* Owner */
 		case 'a': /* Channel admin */
-			if(!is_oper && !is_machine && !is_owner) {
+			if(!is_oper && !is_machine && !is_owner && !is_admin) {
 				connected = IRC_WriteStrClient(Origin,
-					ERR_CHANOPRIVSNEEDED_MSG,
+					ERR_CHANOPPRIVTOOLOW_MSG,
 					Client_ID(Origin),
 					Channel_Name(Channel));
 				goto chan_exit;