Added extra checks for invalid file chars in newfile.php and newfolder.php

- added PHP utility function to check for file name validity
- fixes issue where a user can create a file called ".." from the files UI
- added extra checks to make sure newfile.php and newfolder.php also
  check for invalid characters
Added file name check in webdav connector

- added file name check for the put, rename and setNames() methods which
  throw a "Bad Request" whenever invalid characters are used
- replaced \OC\Filesystem usage with $this->getFS() to be able to write
  unit tests
Merge pull request #6714 from owncloud/files-newfileinvalidcharsfix

Added extra checks for invalid file chars in newfile.php and newfolder.php