parse user configuration before exclusive mode

parse user configuration before exclusive mode

The configuration file was parsed when named was in exclusive
(i.e. single-threaded) mode and unable to answer queries. Because
the parsing is a self-contained operation, it is now done before
named enters exclusive mode.

This reduces the amount of time named can't answer queries when
reloading the configuration when the configuration file is large.
Note that exclusive mode is still used for applying the
configuration changes to the server.

Also, simplify the configuration logic by parsing the built-in
configuration only once at server start time.