[tlbuild] tl18 trials

[Karl Berry]

Hi everyone - there has been no shortage of changes in the TL sources in
the past few weeks, but I think/hope things are converging. At least, I
don't know of anything else myself that is still pending. (I hope there
will be an asymptote update coming, but that's separate anyway.)

So, please give the usual build a try and report problems.  If things go
reasonably well, maybe we'll be able to start the pretest + make the
release a little early.  That would be nice.  --thanks, karl.