pgsql: Add vacuum_freeze_table_age GUC option, to control when VACUUM

Log Message:
-----------
Add vacuum_freeze_table_age GUC option, to control when VACUUM should
ignore the visibility map and scan the whole table, to advance
relfrozenxid.

Modified Files:
--------------
pgsql/doc/src/sgml:
catalogs.sgml (r2.188 -> r2.189)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.188&r2=2.189)
config.sgml (r1.205 -> r1.206)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.205&r2=1.206)
maintenance.sgml (r1.88 -> r1.89)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml?r1=1.88&r2=1.89)
pgsql/src/backend/commands:
cluster.c (r1.180 -> r1.181)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c?r1=1.180&r2=1.181)
vacuum.c (r1.384 -> r1.385)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c?r1=1.384&r2=1.385)
vacuumlazy.c (r1.116 -> r1.117)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c?r1=1.116&r2=1.117)
pgsql/src/backend/nodes:
copyfuncs.c (r1.419 -> r1.420)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c?r1=1.419&r2=1.420)
equalfuncs.c (r1.344 -> r1.345)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c?r1=1.344&r2=1.345)
pgsql/src/backend/parser:
gram.y (r2.654 -> r2.655)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.654&r2=2.655)
pgsql/src/backend/postmaster:
autovacuum.c (r1.91 -> r1.92)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c?r1=1.91&r2=1.92)
pgsql/src/backend/utils/misc:
guc.c (r1.493 -> r1.494)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.493&r2=1.494)
postgresql.conf.sample (r1.252 -> r1.253)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.252&r2=1.253)
pgsql/src/include/catalog:
catversion.h (r1.517 -> r1.518)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.517&r2=1.518)
pg_autovacuum.h (r1.10 -> r1.11)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_autovacuum.h?r1=1.10&r2=1.11)
pgsql/src/include/commands:
vacuum.h (r1.83 -> r1.84)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/vacuum.h?r1=1.83&r2=1.84)
pgsql/src/include/nodes:
parsenodes.h (r1.387 -> r1.388)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.387&r2=1.388)