CONSTANT/NOT NULL/initializer properties for plpgsql record variables

I said a couple of times in recent threads that it wouldn't be too hard
to implement $SUBJECT given the other patches I've been working on.
Attached is proof of the pudding. This needs to be applied on top of
the patches in
https://postgr.es/m/23537.1514589378@sss.pgh.pa.us
and
https://postgr.es/m/11986.1514407114@sss.pgh.pa.us

It's pretty straightforward really. Worth noting is that this also
fixes the null-domain-value issues I mentioned as being a loose end
in the first of the above-referenced messages. Also, I created a
new plpgsql test file for these features, and moved the one relevant
existing test case into that file.

I was a bit disappointed to find that no documentation changes seem
needed, because the SGML docs fail to acknowledge that these cases
didn't work ...

regards, tom lane