Monitoring disk space from within the server

Monitoring the available disk space is the topmost thing on the
priority for PostgreSQL operation, yet this metric is not available
from the SQL level.

The attached patch implements a function pg_tablespace_statfs(tblspc)
to report disk space numbers per tablespace:

# select * from pg_tablespace_statfs('pg_default');
blocks │ bfree │ bavail │ files │ ffree
───────────┼──────────┼──────────┼──────────┼──────────
103179564 │ 20829222 │ 20815126 │ 26214400 │ 24426295

Open points:
* should these numbers be converted to bytes?
* the column names currently mirror the statfs() names and should
certainly be improved
* which of these columns add to \db+ output?
* possibly extend this (and \db) to pg_wal

Christoph