pgsql: Make archiver process report its progress in PS display.

Log Message:
-----------
Make archiver process report its progress in PS display. Per
proposal by Simon Riggs, though not exactly his patch.

Modified Files:
--------------
pgsql/src/backend/postmaster:
pgarch.c (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c?r1=1.35&r2=1.36)