pgsql: Migrate the former contrib/txid module into core.

Log Message:
-----------
Migrate the former contrib/txid module into core. This will make it easier
for Slony and Skytools to depend on it. Per discussion.

Modified Files:
--------------
pgsql/doc/src/sgml:
datatype.sgml (r1.209 -> r1.210)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml?r1=1.209&r2=1.210)
func.sgml (r1.400 -> r1.401)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.400&r2=1.401)
pgsql/src/backend/utils/adt:
Makefile (r1.67 -> r1.68)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/Makefile?r1=1.67&r2=1.68)
pgsql/src/include/catalog:
catversion.h (r1.431 -> r1.432)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.431&r2=1.432)
pg_proc.h (r1.473 -> r1.474)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.473&r2=1.474)
pg_type.h (r1.188 -> r1.189)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_type.h?r1=1.188&r2=1.189)
pgsql/src/include/utils:
builtins.h (r1.304 -> r1.305)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.304&r2=1.305)
pgsql/src/test/regress:
parallel_schedule (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/parallel_schedule?r1=1.44&r2=1.45)
serial_schedule (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/serial_schedule?r1=1.41&r2=1.42)

Added Files:
-----------
pgsql/src/backend/utils/adt:
txid.c (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/txid.c?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/src/test/regress/expected:
txid.out (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/txid.out?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/src/test/regress/sql:
txid.sql (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/txid.sql?rev=1.1&content-type=text/x-cvsweb-markup)