Buildfarm support for older versions

OK, old_branches_of_interest.txt now exists on the buildfarm server, and
the code has been modified to take notice of it (i.e. to accept builds
for branches listed there). The contents are the non-live versions from
9.2 on.

I have set up a test buildfarm client (which will eventually report
under the name 'godwit') alongside crake (Fedora 34). So far testing has
run smoothly, there are only two glitches:

* 9.3 and 9.2 don't have a show_dl_suffix make target. This would
require backpatching b40cb99b85 and d9cdb1ba9e. That's a tiny
change, and I propose to do it shortly unless there's an objection.
* I need to undo the removal of client logic that supported 9.2's
unix_socket_directory setting as opposed to the later
unix_socket_directories.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com