fix strcoll() workaround

The strcoll() workaround we had in place for Kobo devices was (or has
become) ineffective. We had set self.strcoll to nil on Kobo devices -
but this was the instance variable. Setting it to nil effectively makes
the instance variable vanish, so when trying to access it later, it
was not there and got looked up via the metatable - which had the original
reference. Setting it to nil had no effect whatsoever.

We simplify that approach and set the replacement function where before we
had set this to nil.

This is a partial fix for issue #1183 (and explains a comment in issue #686
which says that the old fix did not work).

However, to really fix #1183 - if we want to do so - we would need a collate
function that normalizes uppercase/lowercase before compare.
implement case-insensitive sort, make default and configurable

This will convert any file name to lowercase before doing the comparison.
Note that this will only work for ASCII character range, a full Unicode
aware solution will be much more complicated. And in the end, file names
are byte arrays, not character strings ;-)

fixes #1183.
Merge pull request #1189 from hwhw/master

fix strcoll() workaround