[Bluetooth] Add timestamp support to L2CAP, RFCOMM and SCO

Enable the common timestamp functionality that the network subsystem
provides for L2CAP, RFCOMM and SCO sockets. It is possible to either
use SO_TIMESTAMP or the IOCTLs to retrieve the timestamp of the
current packet.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>