pg_freespacemap: Use SQL-standard function bodies
72a5b1fc8804 was the piece missing for the conversion of this module.
pg_freespace is bumped to 1.3, with its function pg_freespace(regclass)
converted to this new style.
There are other modules in the tree that need a similar treatment; these
will be handled later.
Author: Tom Lane
Reviewed-by: Ronan Dunklau
Discussion: https://postgr.es/m/
3395418.
1618352794@sss.pgh.pa.us