Add logic for detecting non-persistent filesystems being utilized by
workdir which would break the upgrade process upon reboot.

Currently we check for tmpfs and mdmfs.

PR:		195006
Differential Revision:	https://reviews.freebsd.org/D1163
Approved by:	cperciva
Fix geom's "usage" generation to not fabricate usage/help output for any
imaginary class we give it.

Differential Revision:	https://reviews.freebsd.org/D1150
Submitted by:	homerj
Approved by:	pjd