[3.13] gh-132099: Harmonize Bluetooth address handling (GH-132486) (GH-132497)

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 BSD, BTPROTO_HCI now checks address for embedded null.
(cherry picked from commit 1fc1df8dcc7a853b0649bc8df37cd07cbd2b6230)