Prefer LiveCD boot during install media boot, then
set up an extra user for installation.  This user
shall be reached via console or SSH soon enough for
a sort of remote install experience.

PR: https://forum.opnsense.org/index.php?topic=3403

Inspired by: DragonFlyBSD (and we're using its installer anyway!)

(cherry picked from commit 710f00e8)
(cherry picked from commit 8ea003a0)
(cherry picked from commit bde361f7)
(cherry picked from commit d4ecff6f)
(cherry picked from commit 557e7271)
(cherry picked from commit 738f733d)