Add timezone and daylight to time.h.

We already implement tzset(3) and tzname, but daylight and timezone which are part of the same interface were missing. This doesn't actually implement timezone support anymore than we already had, but it will satisfy ports that use the interface (e.g. python, which is currently patched to use hardcoded values instead) and upgrade things to work better in the future.

Should I also modify localtime(3) to take advantage of these, so that it will start working correctly once tzset(3) is extended to have full timezone capabilities, or should that be done separately? How about ports which currently work around lack of daylight and timezone, should I undo the patches or should that be left for when those ports are upgraded?

Closes #803.