Clear up confusion of log_rotate_size config value

Remove the second comment which could be confused with suggesting an invalid value: `// 100 MiB`.

To keep the easily readable example value, it has been moved into the comment header for the config item.
dn needs to be fetched to be able to detect memberOf support
Close the session for preview generation

Without closing the session every preview image generation is locking the session which makes the webinterface unresponsive.
Inject config into checkserver and cleanup tests

Conflicts:
	lib/private/util.php
Don't complain about non-writable datadirs before we're installed

Conflicts:
	lib/private/util.php
Check for writable datadir during setup
Don't test for htaccess if we cant write into the datadir anyway
Try to create datadirectory to test whether the .htaccess works

Fixes https://github.com/owncloud/core/pull/11299#issuecomment-56968588 and also https://github.com/owncloud/core/issues/10628 together with https://github.com/owncloud/core/pull/11299#issuecomment-56968588 when backported.