Auto-enable manteinance mode on server updates

[SamuAlfageme]

If you happen to have a desktop sync client open during the updater's operation it can easily go mad if the server doesn't advertise at any point that is going trough some special condition (such as replacing its own code).

Current Behavior:

At some point,

curl 'https://<server>/status.php' \
    -H 'Authorization:Basic ...' \
    -H 'User-Agent:Mozilla/5.0 (Macintosh) mirall/2.5.0-nightly20171030' \
    [...]'

gets replied back and forth with a 302 redirection to https://<server>/index.php and eventually goes 401:

Expected Behavior:

Receiving a 503 and properly setting the maintenance value on status.php will help the client handle this situation on a more graceful manner (see owncloud/client#5755) - This will require, keeping status.php available during upgrade.

cc/ @DeepDiver1975 opinions?