mktime: improve heuristic for ca-1986 Indiana DST

This patch syncs mktime.c from Gnulib, fixing a
problem reported by Mark Krenz <https://bugs.gnu.org/48085>,
and it should fix BZ#29035 too.
* time/mktime.c (__mktime_internal): Be more generous about
accepting arguments with the wrong value of tm_isdst, by falling
back to a one-hour DST difference if we find no nearby DST that is
unusual.  This fixes a problem where "1986-04-28 00:00 EDT" was
rejected when TZ="America/Indianapolis" because the nearest DST
timestamp occurred in 1970, a temporal distance too great for the
old heuristic.  This also also narrows the search a bit, which
is a minor performance win.

(cherry picked from commit 83859e11)