mktime: improve tm_isdst heuristic

mktime: improve tm_isdst heuristic

* lib/mktime.c (__mktime_internal): When tm_isdst disagrees with
what was requested, search at most a year (+ stride) from the
requested time for a matching tm_isdst, and ignore the request if
the search fails.  This is more likely to match user expectations
in timezones like Asia/Kolkata.
Problem reported by Florian Weimer in:
https://sourceware.org/pipermail/libc-alpha/2025-January/163342.html