MFC r329092:
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
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8801