Add the SockaddrRFCOMM type and the ability to accept BTPROTO_L2CAP and BTPROTO_RFCOMM protocol sockets.

Fixes golang/go#22211

Change-Id: I580b526c5392b90663499af0f2ebe6f55b1f8154
Reviewed-on: https://go-review.googlesource.com/122457
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>