mktime: fix non-EOVERFLOW errno handling

[BZ#23789]
mktime was not properly reporting failures when the underlying
localtime_r fails with errno != EOVERFLOW; it incorrectly treated
them like EOVERFLOW failures, and set errno to EOVERFLOW.
The problem could happen on non-glibc platforms, with Gnulib.
* time/mktime.c (guess_time_tm): Remove, replacing with ...
(tm_diff): ... this simpler function, which does not change errno.
All callers changed to deal with errno themselves.
(ranged_convert, __mktime_internal): Return failure immediately if
the underlying function reports any failure other than EOVERFLOW.
(__mktime_internal): Set errno to EOVERFLOW if the spring-forward
gap code fails.