psql: Improve tab completion for large objects.

psql: Improve tab completion for large objects.

This commit enhances psql's tab completion support for large objects:
- Completes \lo_export <oid> with a file name
- Completes GRANT/REVOKE ... LARGE with OBJECT
- Completes ALTER DEFAULT PRIVILEGES GRANT/REVOKE ... LARGE with OBJECTS

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Chao Li <lic@highgo.com>
Discussion: https://postgr.es/m/87y0syikki.fsf@wibble.ilmari.org