cephfs: add ceph-dokan unmap command

cephfs: add ceph-dokan unmap command

At the moment, Windows CephFS mounts can only be removed by
terminating the daemon (e.g. sending CTRL-C) or through the
Windows mount manager if the "-o -m" parameters were passed
when the mapping was created.

This change adds the "ceph-dokan unmap" command, which takes
the mountpoint as input.

Fixes: https://tracker.ceph.com/issues/49662
Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>
(cherry picked from commit cd9b098afd2d6af3c097c9f79a6a3f924de9e02b)