Code review for contrib/pg_freespacemap. Add a storedpages column to

Code review for contrib/pg_freespacemap.  Add a storedpages column to
pg_freespacemap_relations --- while one could theoretically get that
number by counting rows in pg_freespacemap_pages, it's surely the hard
way to do it.  Avoid expensive and inconvenient conversion to and from
text format.  Minor code and docs cleanup.