Add some socket examples of bidirectional command helpers and pexpect

bidir_cmd_herlper.sh: Bidirectional command helper by socat.
bidir_cmd_helper.py: Bidirectional command helper by python and netcat.

pexpect-ping.py, pexpect-pong.py: Scripts for throwing Ping-Pong between
cross-connected tios.

examples/socket/pexpect-ping.py

@@ -0,0 +1,22 @@
+#!/usr/bin/python3
+# In MSYS2, use /mingw64/bin/python3
+#
+# Send a "Ping" and wait for a "Pong".
+# Repeat this process.
+#
+
+import pexpect
+from pexpect import popen_spawn
+
+child = pexpect.popen_spawn.PopenSpawn("nc -UN /tmp/tio-socket0")
+
+cnt = 0
+while True:
+    try:
+        child.sendline(f"Ping {cnt:d}")
+        cnt += 1
+        child.expect(r'Pong \d+[\r\n]+', timeout = 10)
+    except Exception as e:
+        print(type(e))
+        break
+