fix several fallouts from r320156, ZFS ABD import

All of the problems were related to the FreeBSD-only features.
One was caused by a mismerge in the zfsbootcfg support code.
All others were in the TRIM support code.

Reported by: ken,

		O. Hartmann <ohartmann@walstatt.org>,
		Trond Endrestøl <Trond.Endrestol@fagskolen.gjovik.no>

MFC after: 1 week
X-MFC with: r320156