Bluetooth: Add global deferred socket parameter

The L2CAP and RFCOMM applications require support for authorization
and the ability of rejecting incoming connection requests. The socket
interface is not really able to support this.

This patch does the ground work for a socket option to defer connection
setup. Setting this option allows calling of accept() and then the
first read() will trigger the final connection setup. Calling close()
would reject the connection.

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