BUG/MINOR: checks: successful check completion must not re-enable MAINT servers

If a server is switched to maintenance mode while a check is in progress,
the successful completion of the check must not switch it back up. This
is still a consequence of using the same function set_server_up() for
every state change. Bug reported by Igor at owind.

This fix should be backported to 1.4 which is affected as well.