bsdinstall: rename "Live CD" to "Live System"

This uses more generic wording in the FreeBSD installer, when offering
the user to use the system booted without running the installer.

The updated wording is also reflected in freebsd-version(1).

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D43233

(cherry picked from commit 19bb1886be4f366e922f0be359874683a243624a)