(__mktime_internal): When the requested time falls

(__mktime_internal): When the requested time falls
in a spring-forward gap of size DT, return a time that is DT
away from the requested time, preferring a time whose tm_isdst
differs from the requested value.  Bump the max number of
probes from 4 to 6 to account for the extra probes needed to
discover a spring-forward gap in the worst case.