Add deprecation notice to load* functions

This functions are deprecated and/or removed since ownCloud 7. Additionally a issubdirectory check has been added here to prevent developers to use this function in a potentially insecure way.

Please review @karlitschek and others. Backport to stable5 and master requested.
Merge pull request #9033 from owncloud/validate-dire

Add deprecation notice to load* functions