fix POSIX-format TZ dst transition times for southern hemisphere

fix POSIX-format TZ dst transition times for southern hemisphere

the time of day at which daylight time switches over is specified in
local time in the dst state prior to the transition. the code for
handling this wrongly assumed it needed to switch whether dst or
standard offset is applied to the transition time when the dst end
date is before the dst start date (souther hemisphere summer), but in
fact the end transition time should always be adjusted for dst, and
the start transition time should always be adjusted for standard time.