bdev: Add verification for file storage paths

An earlier version of this patch series verified all paths in cmdlib in
the master daemon. With this change all that verification code is moved
to bdev to run inside the node daemon. The checks are much stricter
now--it is no longer possible to use forbidden paths (e.g. /bin) to
manipulate file storage devices (once these checks are being used).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>