vfs: add a special option to hide dcache from dcap clients

applications built with libdcap can detect dCache through
mounted NFS and will switch to dcap. To enforce NFS access
hide dCache from them.

target: master, 0.7
Acked-by: Paul Millar
(cherry picked from commit 1c916c5fb5476aac2e8445a30d92832e8b156caa)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>