Add -o [no]verify option to mdconfig (and document in man page.)
Implement GEOM attribute MNT::verified to ask md if the backing vnode is
verified.
Check for MNT::verified in cd9660 mount to flag the mount as MNT_VERIFIED if
the underlying device has been verified.
Reviewed by: rwatson
Approved by: sjg (mentor)
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D2902
9a81ba0f24