Add a deprecation warning when using the feature which mounts devices
to see how much space it on them.

Adjust MOUNT_CHAR_DEVS to allow the free space of already mounted
devices to be displayed and report an appropriate error if the
device isn't mounted.

Reviewed by: cem
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8801