bsdinstall: Add "Finish" button to finalconfig

This adds a "Finish" button to the finalconfig step, replacing the
previous first choice, which was to "apply configuration and exit".
The new button is the default action, while the "OK" button is renamed
to "Select".

Also, if an unknown option is chosen, the code flow prompts the user
again instead of exiting.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42047