limit search to files by adding an additional mount point there

@icewind1991 @andrewsbrown this PR supersedes https://github.com/owncloud/core/pull/4918
The search SQL uses the storage id, so I mount a separate locla storage for the users files folder.
@icewind1991 this fixes the problem of finding results outside "/files" in effect really jailing the user to that folder. You like this app
@andrewsbrown could you test search results for files external and shared files? you can use webdav with http://demo.owncloud.org/ 
@schiesbn trashcan still works as expected
sharing works as well.
@georgehrke removed a hack that truncated the 'files/' prefix in the file helper