Fix ESP generation when using a gmirror, and when booting from RO medium

When using a gmirror, entries in /dev can be removed. So instead of using
kern.disks, get the list of disks from "gpart status -sg" instead.

We assume that any 'efi' partition that can't be mounted as msdosfs should
be used as an ESP. However, the ESP on the CD/DVD can't be mounted read-write
and so was being treated as if unformatted. Try the mount as read-only
instead, to catch cases like this.

Relnotes: yes
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D18645