gh-94821: Fix autobind of empty unix domain address (GH-94826)

When binding a unix socket to an empty address on Linux, the socket is
automatically bound to an available address in the abstract namespace.

    >>> s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    >>> s.bind("")
    >>> s.getsockname()
    b'\x0075499'

Since python 3.9, the socket is bound to the one address:

    >>> s.getsockname()
    b'\x00'

And trying to bind multiple sockets will fail with:

    Traceback (most recent call last):
      File "/home/nsoffer/src/cpython/Lib/test/test_socket.py", line 5553, in testAutobind
        s2.bind("")
    OSError: [Errno 98] Address already in use

Added 2 tests:
- Auto binding empty address on Linux
- Failing to bind an empty address on other platforms

Fixes f6b3a07b7d (bpo-44493: Add missing terminated NUL in sockaddr_un's length (GH-26866)

Modules/socketmodule.c

@@ -1724,8 +1724,10 @@ getsockaddrarg(PySocketSockObject *s, PyObject *args,
 
         struct sockaddr_un* addr = &addrbuf->un;
 #ifdef __linux__
-        if (path.len > 0 && *(const char *)path.buf == 0) {
-            /* Linux abstract namespace extension */
+        if (path.len == 0 || *(const char *)path.buf == 0) {
+            /* Linux abstract namespace extension:
+               - Empty address auto-binding to an abstract address
+               - Address that starts with null byte */
             if ((size_t)path.len > sizeof addr->sun_path) {
                 PyErr_SetString(PyExc_OSError,
                                 "AF_UNIX path too long");