Add LargeFileHelper / Add CURL filesize workaround / Fix some 32-bit filesize issues

Fixes #4979, #6516 and possibly others of
https://github.com/owncloud/core/issues?labels=php%3A32-bit-integer&state=open

Determination of file size on 32-bit systems (curl extension highly recommended).
Corrects the display of used and available storage in "Personal".
Corrects the reported filesize that is reported via WebDAV.