[tex-live] TL'10 pretest trial

[Karl Berry]

    need to do anything, and each user wanting to use MkIV would need to generate
    her own cache herself. Is that ok for you?

It's what we've done before, and what I expected to do this year.
I'd rather not attempt to call luatools --generate as part of install-tl.

    ~/.texlive2010 already contains two subdirectories: texmf-var and texmf-config.

I propose $TEXMFVAR/texmf-var/luatex-cache.  I see no advantage to
proliferating directories under ~/.texliveYYYY.  And texmf-var is
already supposed to be only stuff that can be regenerated, which this is.
?