pgsql: Cause the archiver process to adopt new postgresql.conf settings

Log Message:
-----------
Cause the archiver process to adopt new postgresql.conf settings (particularly
archive_command) as soon as possible, namely just before issuing a new call
of archive_command, even when there is a backlog of files to be archived.
The original coding would only absorb new settings after clearing the backlog
and returning to the outer loop.  Per discussion.

Back-patch to 8.3.  The logic in prior versions is a bit different and it
doesn't seem worth taking any risks of breaking it.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        pgarch.c (r1.41 -> r1.42)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c?r1=1.41&r2=1.42)