Assert ngIRCd starts with UID/GID "irc"

This restores the behaviour of ngircd up to 15-0.1, so ngIRCD will run
as "irc" even if UID/GID are not configured. At the price of warning
messages, so document them.

debian/ngircd.README.Debian

@@ -6,6 +6,23 @@ Generic
 For generic information, including the new names of configuration
 variables, see /usr/share/doc/ngircd/INSTALL.gz
 
+Debian specific
+---------------
+The configuration of the user and group ID ngIRCd runs as has
+changed in ngircd 18-1. If you see messages like
+
+    Can't change group ID to 65534: Operation not permitted
+
+Change the lines
+
+    ;ServerUID = 65534
+    ;ServerGID = 65534
+
+into
+
+    ServerGID = irc
+    ServerUID = irc
+
 
 TLS support
 ===========

debian/ngircd.init

@@ -63,6 +63,7 @@ do_start()
 	--pidfile "$PIDFILE" --exec "$DAEMON" \
 	--test > /dev/null || return 1
     start-stop-daemon --start --quiet \
+	--chuid "$DAEMONUSER:$DAEMONUSER" \
 	--pidfile "$PIDFILE" --exec "$DAEMON" -- \
 	$DAEMON_ARGS || return 2
     # Add code here, if necessary, that waits for the process to be ready