Tab completion for large objects

Hi hackers,

I noticed that psql's tab completion suggested TO immediately after
GRANT ... ON LARGE OBJECT, and not after ON LARGE OBJECT <oid>. This is
because LARGE OBJECT is the only two-word object type, so it thinks
LARGE is the object type and OBJECT is the name. Attached are three
patches that address this and other LO-related tab completion issues:

1. Tab complete OBJECT after GRANT|REVOKE ... ON LARGE, and TO/FROM
after GRANT|REVOKE ... ON LARGE OBJECT

2. Tab complete filenames after \lo_export <oid>

3. Tab complete large object OIDs where relevant. This is less useful
than completing names like for other objects, but it might still
be handy. Separate patch in case it proves controversial or is
deemed useless.

- ilmari