pgsql: Fix handling of autovacuum reloptions.

Log Message:
-----------
Fix handling of autovacuum reloptions.

In the original coding, setting a single reloption would cause default
values to be used for all the other reloptions.  This is a problem
particularly for autovacuum reloptions.

Itagaki Takahiro

Modified Files:
--------------
    pgsql/src/backend/access/common:
        reloptions.c (r1.28 -> r1.29)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/reloptions.c?r1=1.28&r2=1.29)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.102 -> r1.103)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.102&r2=1.103)