OStatus fixup-shadow.php: add check for user_group entries with 'uri' entry shadowing...

OStatus fixup-shadow.php: add check for user_group entries with 'uri' entry shadowing an actual local_group -- this can cause trouble now that we sometimes actually use the uri field for lookups, and leaving the entries around would break updating the table to fill out formerly missing uris, since the unique index entry would already be taken by the bad entry.