Fix ranges with multibyte characters as endpoints.

Fix ranges with multibyte characters as endpoints.

This is another bug in computing the fastmap.  It was reported by a user
of sed because it usually does not happen with !_LIBC.  However, it is
there in that case too.

The bug is that whenever we have a range at the beginning of the regex,
the regex must be tested on any possible multibyte character.  The reason
why _LIBC masks it, is that in general there is a collation symbol for
each possible multibyte-character lead byte, so all the lead bytes are
in general already part of the fastmap.

The tests use cyrillic characters as an example.  With _LIBC, they pass
without the patch too, but you can make them fail by removing collation
symbols handling.