Adjust postgres_fdw's search path handling.

Set the remote session's search path to exactly "pg_catalog" at session
start, then schema-qualify only names that aren't in that schema.  This
greatly reduces clutter in the generated SQL commands, as seen in the
regression test changes.  Per discussion.

Also, rethink use of FirstNormalObjectId as the "built-in object" cutoff
--- FirstBootstrapObjectId is safer, since the former will accept
objects in information_schema for instance.
Fix thinko in previous commit.

We must still initialize minRecoveryPoint if we start straight with archive
recovery, e.g when recovering from a normal base backup taken with
pg_start/stop_backup. Otherwise we never consider the system consistent.