gh-132099: Harmonize Bluetooth address handling (GH-132486)

Now all protocols always accept the Bluetooth address as string and
getsockname() always returns the Bluetooth address as string.

* BTPROTO_SCO now accepts not only bytes, but str.
* BTPROTO_SCO now checks address for embedded null.
* On *BSD, BTPROTO_HCI now accepts str instead of bytes.
* On FreeBSD, getsockname() for BTPROTO_HCI now returns str instead of bytes.
* On NetBSD and DragonFly BDS, BTPROTO_HCI now checks address for embedded null.