pgsql: Add relation fork support to pg_relation_size() function.

Log Message:
-----------
Add relation fork support to pg_relation_size() function. You can now pass
name of a fork ('main' or 'fsm', at the moment) to pg_relation_size() to
get the size of a specific fork. Defaults to 'main', if none given.

While we're at it, modify pg_relation_size to take a regclass as argument,
instead of separate variants taking oid and name. This change is
transparent to typical use where the table name is passed as a string
literal, like pg_relation_size('table'), but will break queries like
pg_relation_size(namecol), where namecol is of type name. text-type input
still works, and using a non-schema-qualified table name is not very
reliable anyway, so this is unlikely to break anyone's queries in practice.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.447 -> r1.448)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.447&r2=1.448)
pgsql/src/backend/utils/adt:
dbsize.c (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/dbsize.c?r1=1.20&r2=1.21)
pgsql/src/include/catalog:
catversion.h (r1.490 -> r1.491)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.490&r2=1.491)
pg_proc.h (r1.515 -> r1.516)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.515&r2=1.516)
pgsql/src/include/storage:
relfilenode.h (r1.17 -> r1.18)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/relfilenode.h?r1=1.17&r2=1.18)
pgsql/src/include/utils:
builtins.h (r1.320 -> r1.321)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.320&r2=1.321)