Allow cluster owner to bypass authentication

This is an implementation of the idea I mentioned in [0].

The naming and description perhaps isn't ideal yet but it works in
principle.

The idea is that if you connect over a Unix-domain socket and the local
(effective) user is the same as the server's (effective) user, then
access should be granted immediately without any checking of
pg_hba.conf. Because it's "your own" server and you can do anything you
want with it anyway.

I included an option to turn this off because (a) people are going to
complain, (b) you need this for the test suites to be able to test
pg_hba.conf, and (c) conceivably, someone might want to have all access
to go through pg_hba.conf for some auditing reasons (perhaps via PAM).

This addresses the shortcomings of using peer as the default mechanism
in initdb. In a subsequent step, my idea would be to make the default
initdb authentication setup to use md5 (or scram, tbd.) for both local
and host.

[0]:
https://www.postgresql.org/message-id/29164e47-8dfb-4737-2a61-e67a18f847f3%402ndquadrant.com

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services