Integrate pg_upgrade_support module into backend

Previously, these functions were created in a schema "binary_upgrade",
which was deleted after pg_upgrade was finished.  Because we don't want
to keep that schema around permanently, move them to pg_catalog but
rename them with a binary_upgrade_... prefix.

The provided functions are only small wrappers around global variables
that were added specifically for pg_upgrade use, so keeping the module
separate does not create any modularity.

The functions still check that they are only called in binary upgrade
mode, so it is not possible to call these during normal operation.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>