mktime: Improve timezone support, increase performance, minor bugfixes.
I am presuming that by now all systems we run on either have the tm_gmtoff
or the __tm_gmtoff struct member on the <time.h> struct tm.
This allows me to squash a few annoying bugs in the System.TM module
and also in the mktime() implementation. It speeds up mktime() a lot,
gets rid of large pieces of stale code, as well as extends its range
to basically unlimited, and even though it improves it a bit, it still
leaves System.TM more limited/buggy/slower than mktime() (especially
when using timezones other than UTC or the local timezone).