Fix mktime localtime offset confusion

[BZ #23603]
* include/time.h (__mktime_internal): The localtime offset is now
of type long int instead of time_t.  This is the longstanding type
in glibc, and it is more than enough to represent difference
between localtime and gmtime even if it is 32 bits and time_t is
64.  Changing it now will let us avoid an unnecessary change when
time_t is widened to 64 bits on 32-bit platforms.
* time/mktime-internal.h (mktime_offset_t): Now long int.