Bluetooth: Add support for HCI monitor channel

The HCI monitor channel can be used to monitor all packets and events
from the Bluetooth subsystem. The monitor is not bound to any specific
HCI device and allows even capturing multiple devices at the same time.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>