Suppress error message which would send headers for hosted sited where disk_free_space() has been disabled for security.

apps/files_external/lib/config.php
apps/files_versions/lib/versions.php
lib/connector/sabre/client.php

Only use a ETag, but include last modified time into this
Also added the filesize to this. And used the ETag for the
internal cache.

Support for unshare from self, with a bunch of temporary fixes to overcome configuration problems with file actions

prefix.

Fixes: oc-1663