install-tl: Changing trailing directory of TEXDIR makes TEXMFLOCAL release specific

[Karl Berry]

    Yes, we only can code as many cases, and we decided to ONLy do the trick
    when the last path component is YYYY, but not when it is anything else.

That was fair enough of us :), but we could make "YYYYbasic" another
special case since it is somehow a known variation. Anyone using both
basic and "regular" mactex almost surely wants a single texmf-local.

Alternatively ... Dick, how does basictex installation actually work?
Sorry, I forget. Do you define a texlive "profile" for install-tl?
Or run install-tl (I doubt it)? In either of those cases, it would be
easy to also change TEXMFLOCAL as part of the installation.

Thanks,
Karl