vt: add US International keymap

This adds support for the US International keyboard layout to vt. It
uses dead keys to generate diacritics such as accents, cedillas and
umlauts.

PR: 72465
Co-authored-by: Helge Oldach <freebsd@oldach.net>
Reviewed-by: Serjei Mikailov <serjeimikailov19@proton.me>
Reviewed-by: Warner Losh <imp@FreeBSD.org>
MFC-after: 2 weeks
Pull-Request: https://github.com/freebsd/freebsd-src/pull/1601