New timing infrastructure. There's a new function schedule_timer()

which pretty much any module can call to request a call-back in the
future. So terminal.c can do its own handling of blinking, visual
bells and deferred screen updates, without having to rely on
term_update() being called 50 times a second (fixes: pterm-timer);
and ssh.c and telnet.c both invoke a new module pinger.c which takes
care of sending keepalives, so they get sent uniformly in all front
ends (fixes: plink-keepalives, unix-keepalives).

[originally from svn r4906]
[this svn revision also touched putty-wishlist]

windows/winstuff.h

@@ -150,6 +150,8 @@ extern int (WINAPI *p_WSAGetLastError)(void);
 extern int (WINAPI *p_WSAEnumNetworkEvents)
     (SOCKET s, WSAEVENT hEventObject, LPWSANETWORKEVENTS lpNetworkEvents);
 
+extern int socket_writable(SOCKET skt);
+
 /*
  * Exports from winctrls.c.
  */