bsdinstall: Attempt setting the date with NTP

[des]

Summary

Before asking the user to set the date and time manually, offer to set
them automatically. We default to freebsd.pool.ntp.org, but give the
user a chance to specify a different server.

Additionally, rewrite the time-setting code so it can run both pre- and
post-install. When running post-install, it checks to see if it has
already run, and if so, retrieves and installs the previously selected
time zone.

Finally, run the time-setting code pre-install if network configuration
has been run, which strongly suggests that we're going to attempt to
download something, and therefore may need to have the right time in
order to validate certificates.

Event: Oslo Hackathon 202508

[jrtc27]

This does too many things in one change. Inferring whether it's pre- or post-install via looking at BSDINSTALL_CHROOT adds unnecessary complexity when the caller of the script already knows if it's pre-install or post-install. D52196 maintains that intentionality, and also better handles the time zone used during the install with a working /etc/localtime rather than needing to mess about with TZ. There may be ways you think the implementation there could be better, but I believe that the general design of it is better.