os_files_private handler shouldn't clobber pinserver's handler #4202

Only enable OSFIlesPrivateSteamWrapper when OSPinserverPrivateStreamWrapper isn't enabled.  This wasn't an issue locally, but hwpi had different module weights.  Instead of changing those, lets just be careful not to force the wrong wrapper.
Change pinserver stream wrapper parent to os files priv #4202
Merge pull request #4226 from openscholar/4202-priv_file_urls

os_files_private handler shouldn't clobber pinserver's handler #4202