Make REPLICATION privilege checks test current user not authenticated user.

The pg_start_backup() and pg_stop_backup() functions checked the privileges
of the initially-authenticated user rather than the current user, which is
wrong.  For example, a user-defined index function could successfully call
these functions when executed by ANALYZE within autovacuum.  This could
allow an attacker with valid but low-privilege database access to interfere
with creation of routine backups.  Reported and fixed by Noah Misch.

Security: CVE-2013-1901