bootstrap pg_shseclabel in relcache initialization

Hi All,

While working on an auth hook, I found that I was unable to access the
pg_shseclabel system table while processing the hook. I discovered
that the only tables that were bootstrapped and made available at this
stage of the the auth process were pg_database, pg_authid and
pg_auth_members. Unfortunately, this is problematic if you have
security labels that are associated with a role which are needed to
determine auth decisions/actions.

Given that the shared relations currently exposed can also have
security labels that can be used for auth purposes, I believe it makes
sense to make those available as well. I have attached a patch that
adds this functionality for review/discussion. If this functionality
makes sense I'll add it to the commitfest.

Thanks,
Adam