pgsql-server: GUC assign hooks that look at external state in deciding

Log Message:
-----------
GUC assign hooks that look at external state in deciding whether a
setting is valid must ignore that state and permit the assignment anyway
when source is PGC_S_OVERRIDE.  Otherwise they may disallow a rollback
at transaction abort, which is The Wrong Thing.  Per example from
Michael Fuhr 12-Sep-04.

Modified Files:
--------------
    pgsql-server/src/backend/commands:
        variable.c (r1.103 -> r1.104)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/variable.c.diff?r1=1.103&r2=1.104)
    pgsql-server/src/backend/utils/misc:
        guc.c (r1.238 -> r1.239)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/guc.c.diff?r1=1.238&r2=1.239)