mktime: systematically normalize tm_isdst comparisons

mktime: systematically normalize tm_isdst comparisons

* lib/mktime.c (isdst_differ): New function.
(__mktime_internal): Use it systematically for all isdst comparisons.
This completes the fix for libc BZ #6723, and removes the need for
normalizing tm_isdst.  See
<http://sourceware.org/bugzilla/show_bug.cgi?id=6723>
(not_equal_tm) [DEBUG]: Use isdst_differ here, too.