Add a few new patterns to the tab completion of psql

Add a few new patterns to the tab completion of psql

This improves the tab completion of psql on a few points:
- Provide a list of subscriptions on \dRs.
- Provide a list of publications on \dRp.
- Add CURRENT_ROLE, CURRENT_USER, SESSION_USER when OWNER TO is provided
at the end of a query (as defined by RoleSpec in gram.y).

Author: Vignesh C
Reviewed-by: Dagfinn Ilmari Mannsåker
Discussion: https://postgr.es/m/CALDaNm3toRBt6c6saY3174f7CsGztXRvVvfWbikkJEXY7x5WAA@mail.gmail.com