pgsql: Code review for contrib/pg_freespacemap.

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/contrib/pg_freespacemap:
        README.pg_freespacemap (r1.3 -> r1.4)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_freespacemap/README.pg_freespacemap.diff?r1=1.3&r2=1.4)
        pg_freespacemap.c (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_freespacemap/pg_freespacemap.c.diff?r1=1.4&r2=1.5)
        pg_freespacemap.sql.in (r1.4 -> r1.5)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_freespacemap/pg_freespacemap.sql.in.diff?r1=1.4&r2=1.5)