pgsql: Simplify initdb-time assignment of OIDs as I proposed yesterday,

Log Message:
-----------
Simplify initdb-time assignment of OIDs as I proposed yesterday, and
avoid encroaching on the 'user' range of OIDs by allowing automatic
OID assignment to use values below 16k until we reach normal operation.

initdb not forced since this doesn't make any incompatible change;
however a lot of stuff will have different OIDs after your next initdb.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        bki.sgml (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/bki.sgml.diff?r1=1.14&r2=1.15)
    pgsql/src/backend/access/transam:
        varsup.c (r1.62 -> r1.63)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/varsup.c.diff?r1=1.62&r2=1.63)
        xlog.c (r1.183 -> r1.184)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.183&r2=1.184)
    pgsql/src/backend/catalog:
        genbki.sh (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/genbki.sh.diff?r1=1.33&r2=1.34)
    pgsql/src/backend/storage/lmgr:
        lock.c (r1.148 -> r1.149)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.148&r2=1.149)
    pgsql/src/backend/utils/misc:
        guc.c (r1.258 -> r1.259)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.258&r2=1.259)
    pgsql/src/bin/pg_resetxlog:
        pg_resetxlog.c (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_resetxlog/pg_resetxlog.c.diff?r1=1.30&r2=1.31)
    pgsql/src/include/access:
        transam.h (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h.diff?r1=1.53&r2=1.54)
    pgsql/src/include/catalog:
        duplicate_oids (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/duplicate_oids.diff?r1=1.5&r2=1.6)
        unused_oids (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/unused_oids.diff?r1=1.5&r2=1.6)