[tlbuild] casefolding fallback search

[Karl Berry]

(Sorry, I was offline for a couple of days.)

I've commented out the failing tests in kpsewhich.test:

#TEXINPUTS=$srcdir/../tests/texmf \
#  ./kpsewhich --no-casefold-search --all Plain.TeX && exit 1
#TEXINPUTS=$srcdir/../tests/texmf \
#  ./kpsewhich --no-casefold-search Plain.TeX && exit 1

And clarified in the documentation (not committed yet) that
--no-casefold-search merely means "kpathsea doesn't do any case folding,
you get whatever the system does".  --thanks, karl.