pgsql: Allow extracting and parsing of reloptions from a bare pg_class

Log Message:
-----------
Allow extracting and parsing of reloptions from a bare pg_class tuple, and
refactor the relcache code that used to do that.  This allows other callers
(particularly autovacuum) to do the same without necessarily having to open
and lock a table.

Modified Files:
--------------
    pgsql/src/backend/access/common:
        reloptions.c (r1.18 -> r1.19)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/reloptions.c?r1=1.18&r2=1.19)
    pgsql/src/backend/utils/cache:
        relcache.c (r1.282 -> r1.283)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c?r1=1.282&r2=1.283)
    pgsql/src/include/access:
        reloptions.h (r1.10 -> r1.11)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/reloptions.h?r1=1.10&r2=1.11)