Add labels to help make psql's hidden queries more understandable.

Add labels to help make psql's hidden queries more understandable.

We recommend looking at psql's "-E" output to help understand the
system catalogs, but in some cases (particularly table displays)
there's a bunch of rather impenetrable SQL there.  As a small
improvement, label each query issued by describe.c with a short
description of its purpose.  The code is arranged so that the
labels also appear as SQL comments in the server log, if the
server is logging these commands.

We could expand this policy to every use of PSQLexec(), but most of
the ones outside describe.c are issuing simple commands like "BEGIN"
or "COMMIT", which don't seem to need such glosses.  I did add
labels to the commands issued by \sf, \sv and friends.

Also, make the -E and log output for hidden queries say
"INTERNAL QUERY" not just "QUERY", to distinguish them from
user-written queries.

Author: Greg Sabino Mullane <htamfids@gmail.com>
Co-authored-by: David Christensen <david+pg@pgguru.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAKAnmmJz8Hh=8Ru8jgzySPWmLBhnv4=oc_0KRiz-UORJ0Dex+w@mail.gmail.com