Available disk space per tablespace

Hi,

I'm picking up a 5 year old patch again:
https://www.postgresql.org/message-id/flat/20191108132419.GG8017%40msg.df7cb.de

Users will be interested in knowing how much extra data they can load
into a database, but PG currently does not expose that number. This
patch introduces a new function pg_tablespace_avail() that takes a
tablespace name or oid, and returns the number of bytes "available"
there. This is the number without any reserved blocks (Unix, f_avail)
or available to the current user (Windows).

(This is not meant to replace a full-fledged OS monitoring system that
has much more numbers about disks and everything, it is filling a UX
gap.)

Compared to the last patch, this just returns a single number so it's
easier to use - total space isn't all that interesting, we just return
the number the user wants.

The free space is included in \db+ output:

postgres =# \db+
List of tablespaces
Name │ Owner │ Location │ Access privileges │ Options │ Size │ Free │ Description
────────────┼───────┼──────────┼───────────────────┼─────────┼─────────┼────────┼─────────────
pg_default │ myon │ │ ∅ │ ∅ │ 23 MB │ 538 GB │ ∅
pg_global │ myon │ │ ∅ │ ∅ │ 556 kB │ 538 GB │ ∅
spc │ myon │ /tmp/spc │ ∅ │ ∅ │ 0 bytes │ 31 GB │ ∅
(3 rows)

The patch has also been tested on Windows.

TODO: Figure out which systems need statfs() vs statvfs()

Christoph