patch 8.2.4684: cannot open a channel on a Unix domain socket

Problem:    Cannot open a channel on a Unix domain socket.
Solution:   Add Unix domain socket support. (closes #10062)

src/testdir/test_channel_unix.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python
+#
+# Server that will accept connections from a Vim channel.
+# Used by test_channel.vim.
+#
+# This requires Python 2.6 or later.
+
+from __future__ import print_function
+from test_channel import ThreadedTCPServer, ThreadedTCPRequestHandler, \
+    writePortInFile
+import socket
+import threading
+import os
+
+try:
+    FileNotFoundError
+except NameError:
+    # Python 2
+    FileNotFoundError = (IOError, OSError)
+
+class ThreadedUnixServer(ThreadedTCPServer):
+    address_family = socket.AF_UNIX
+
+def main(path):
+    server = ThreadedUnixServer(path, ThreadedTCPRequestHandler)
+
+    # Start a thread with the server.  That thread will then start a new thread
+    # for each connection.
+    server_thread = threading.Thread(target=server.serve_forever)
+    server_thread.start()
+
+    # Signal the test harness we're ready, the port value has no meaning.
+    writePortInFile(1234)
+
+    print("Listening on {0}".format(server.server_address))
+
+    # Main thread terminates, but the server continues running
+    # until server.shutdown() is called.
+    try:
+        while server_thread.is_alive():
+            server_thread.join(1)
+    except (KeyboardInterrupt, SystemExit):
+        server.shutdown()
+
+if __name__ == "__main__":
+    try:
+        os.remove("Xtestsocket")
+    except FileNotFoundError:
+        pass
+    main("Xtestsocket")