[tlbuild] Binaries in TL source [trunk]

[Karl Berry]

Hi Angelo,

    good to mix binaries and sources

Of course it's not good.  I hope you don't think I was happy about this
approach.  Just the opposite, believe me.  As I wrote in the README, I
just didn't see any other practical way to do it.  It's cost a lot of
effort and time in making the special case, but given how popular biber
+ biblatex have become, it seemed worth it.

    /tlnet/install -biber-bin PATH_TO_BIBER_BIN...

I don't see the advantage to piling more kludges on top of the existing
kludges.  Just install biber independently in your /usr/local/bin.
Sorry about not working well with -custom-bin, but this whole thing has
been a pain from beginning to end.

Best,
Karl