date: add -z output_zone option

Inpired by OpenBSD date(1), this option allows to do timezone conversion
via the date(1) command.

For example, to determine when the BSDCan livestream begins for me:

$ env -i TZ=EST5EDT date -z Europe/Paris -j 0900

MFC After: 1 week
Reviewed by: kib, bcr (manpage)
Differential Revision: https://reviews.freebsd.org/D40159