Allow callers to pass a missing_ok flag when opening a relation.

Since the names try_relation_openrv() and try_heap_openrv() don't seem
quite appropriate, rename the functions to relation_openrv_extended()
and heap_openrv_extended().  This is also more general, if we have a
future need for additional parameters that are of interest to only a
few callers.

This is infrastructure for a forthcoming patch to allow
get_object_address() to take a missing_ok argument as well.

Patch by me, review by Noah Misch.