Fix SIGINT of ejabberdctl foreground by exec(1) on privilege drop

When `ejabberdctl foreground` is executed as root, it will su to the
ejabberd user before starting Erlang.
If a user then hits ^C, su(1) kills the login shell it started and
the Erlang process is interited by PID 1 instead of being stopped.

Prepending the command sent to the ejabberd user's shell with "exec"
fixes this behaviour for me, because su(1) then kills the Erlang process
instead of the login shell.

ejabberdctl.template

@@ -121,7 +121,7 @@ set_dist_client()
 exec_cmd()
 {
     case $EXEC_CMD in
-        as_install_user) su -s /bin/sh -c '"$0" "$@"' "$INSTALLUSER" -- "$@" ;;
+        as_install_user) su -s /bin/sh -c 'exec "$0" "$@"' "$INSTALLUSER" -- "$@" ;;
         as_current_user) "$@" ;;
     esac
 }