Parse pg_hba.conf in postmaster, instead of once in each backend for

Parse pg_hba.conf in postmaster, instead of once in each backend for
each connection. This makes it possible to catch errors in the pg_hba
file when it's being reloaded, instead of silently reloading a broken
file and failing only when a user tries to connect.

This patch also makes the "sameuser" argument to ident authentication
optional.