[Merge] perldiag stuff

I noticed that some entries in perldiag had (F ...) with warnings
categories.  Those categories apply only to warnings, not to
fatal errors.

So I looked to see why diag.t was not already catching it, since I
thought I had programmed it to.

It turned out that it was skipping any function calls that lacked
aTHX_, which included yyerror and anything with _nocontext.

So I fixed that and got pages and pages of failures, unsurprisingly.

I went through, fixing them, and inevitably tweaking other things.

In one case, I even changed the message output, removing an extraneous
dot before ‘at file line 24’.