rbd-nbd: introduce new attach and detach commands

rbd-nbd: introduce new attach and detach commands

The commands allow to restart a daemon without destroying the nbd
device.

Now, if the netlink is used, a dead connection timeout is set on
the nbd device setup, so the device is not immediately released
if the rbd-nbd process terminates without disconnect (unmap).

The attach command just sends terminate signal to the rbd-nbd
process. The detach command starts a new process and connects to
the existing device.

Signed-off-by: Mykola Golub <mgolub@suse.com>