Bluetooth: Preparation for usage of SOL_BLUETOOTH

The socket option levels SOL_L2CAP, SOL_RFOMM and SOL_SCO are currently
in use by various Bluetooth applications. Going forward the common
option level SOL_BLUETOOTH should be used. This patch prepares the clean
split of the old and new option levels while keeping everything backward
compatibility.

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