Refactor routines for name lookups of procedures and operators

Refactor routines for name lookups of procedures and operators

This introduces a new set of extended routines for procedure and
operator name lookups, with a flag bitmask argument that can modify the
result.  The following options are available:
- Force schema qualification, ignoring search_path.  This is similar to
the existing option for format_{operator|procedure}_qualified().
- Force NULL as result instead of a numeric OID for an undefined
object.  This option is new.

This is a refactoring similar to 1185c78, that will be used for a future
patch to improve the SQL functions providing information using object
addresses for undefined objects.

Author: Michael Paquier
Reviewed-by: Aleksander Alekseev, Dmitry Dolgov, Daniel Gustafsson,
Álvaro Herrera
Discussion: https://postgr.es/m/CAB7nPqSZxrSmdHK-rny7z8mi=EAFXJ5J-0RbzDw6aus=wB5azQ@mail.gmail.com