Add pg_relation_by_filenode(reltbspc, filenode) admin function

Now that I proposed a new syscache upthread its easily possible to provide
pg_relation_by_filenode which I wished for multiple times in the past when
looking at filesystem activity and wondering which table does what. You can
sortof get the same result via

SELECT oid FROM (
SELECT oid, pg_relation_filenode(oid::regclass) filenode
FROM pg_class WHERE relkind != 'v'
) map
WHERE map.filenode = ...;

but thats neither efficient nor obvious.

So, two patches to do this:

Did others need this in the past? I can live with the 2nd patch living in a
private extension somewhere. The first one would also be useful for some
error/debug messages during decoding...

Greetings,

Andres