pgsql: Simplify relcache startup sequence.

Log Message:
-----------
Simplify relcache startup sequence.  With the new design of InitPostgres
it's not necessary to have three separate calls anymore.  This patch also
fixes things so we don't try to read pg_internal.init until after we've
obtained lock on the target database; which was fairly harmless, but it's
certainly cleaner this way.

Modified Files:
--------------
    pgsql/src/backend/utils/cache:
        relcache.c (r1.239 -> r1.240)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.239&r2=1.240)
    pgsql/src/backend/utils/init:
        postinit.c (r1.166 -> r1.167)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.166&r2=1.167)
    pgsql/src/include/utils:
        relcache.h (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/relcache.h.diff?r1=1.53&r2=1.54)