Add helper to check for `ini` values in `OC_Util::checkServer`

This allows to check for specific values in the PHP.ini that ownCloud requires for full compatibility.

`mbstring.func_overload`: https://github.com/owncloud/core/issues/14372
`output_buffering`: http://doc.owncloud.org/server/8.0/admin_manual/configuration/big_file_upload_configuration.html#configuring-php

Fixes https://github.com/owncloud/core/issues/14372 and https://github.com/owncloud/core/issues/14412