Add explicit exit command.

Some platforms (notably, Windows) do not support graceful process termination
with SIGINT signal, this change adds an explicit "exit" command so the
server could be stopped gracefully from input command rather than a signal.

tsproxy.py

diff --git a/tsproxy.py b/tsproxy.py
index 2e55aeb..45e4e57 100644
--- a/tsproxy.py
+++ b/tsproxy.py
@@ -615,6 +615,7 @@
     global REMOVE_TCP_OVERHEAD
     global port_mappings
     global server
+    global must_exit
     if len(input):
       ok = False
       try:
@@ -652,6 +653,9 @@
             if command[1].lower() == 'mapports' or command[1].lower() == 'all':
               port_mappings = {}
               ok = True
+          elif command[0].lower() == 'exit':
+              must_exit = True
+              ok = True
 
           if ok:
             needs_flush = True