[tlbuild] pretest initial builds please

[Karl Berry]

As far as I know we are ready to start updating binaries for the TL17
pretest. Last night's update of tlnet was the last for TL16.

Although there will be more changes to the sources, we have to start
somewhere, so please go ahead and do your builds, and commit/tell me
urls as usual.

I have committed new binaries for i386-linux and x86_64-linux.
(Norbert, please overwrite my x64 binaries with yours when you build
them, I just had to have something to test the builds.) I changed
tl-update-bindir to preserve an already-committed dvisvgm binary (like
it preserves asy, biber, and xindy), as we've discussed.

One minor complication at the moment is that gregorio builds as the
executable name gregorio-5_0_0-rc1. I hope the developers will agree to
change this. I manually renamed it to "gregorio" for now, but it is not
necessary for anyone else to bother, until the situation becomes clear.

The Build/source/utils/asymptote sources are current. I'll commit the
new support files to Master soon, probably tomorrow.

Chime in if questions/problems, as always ... --thanks, karl.