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.
Also, add some messages to let users know what the script is doing.
Relnotes: yes
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