date: add -z output_zone option

[bapt]

Summary

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

[bapt]

fix manpage date

[kib]

bin/date/date.c

196

if (outzone != NULL && setenv("TZ", outzone, 1) != 0)
    err(1, "setenv(TZ)");

[bcr]

OK for the man page part.

[bapt]

Address @kib comments

[kib]

I think that the man page ought to include the example from the commit message as well, since an idea to use both -z and TZ is not obvious.

[bapt]

Add an example in the manpage of the usage of -z option