Fix Win32 event use.

Incorrect use of the Ctrl+C event can lead to busy-waiting in socket and
serial port IO.

util/ctrlc.h

@@ -60,6 +60,10 @@ void ctrlc_raise(void);
  * object which becomes signalled when the Ctrl+C event is raised.
  * Implementations of Windows IO operations should allow operations to
  * be interrupted by the signalling of this object.
+ *
+ * The event can be manually cleared before IO operations, but this
+ * doesn't clear the recorded Ctrl+C event. If the event is manually
+ * cleared, the Ctrl+C event status should be checked *after* doing so.
  */
 HANDLE ctrlc_win32_event(void);
 #endif

util/sockets.c

@@ -37,6 +37,11 @@ static void sockets_begin(SOCKET s, DWORD event)
 
 	ioctlsocket(s, FIONBIO, &mode);
 	WSAEventSelect(s, ctrlc_win32_event(), event);
+
+	/* We explicitly check for Ctrl+C after resetting the event in
+	 * sockets_wait().
+	 */
+	ResetEvent(ctrlc_win32_event());
 }
 
 static int sockets_wait(DWORD timeout)

util/sport.c

@@ -256,6 +256,8 @@ static int xfer_wait(sport_t s, LPOVERLAPPED ovl)
 {
 	DWORD result = 0;
 
+	ResetEvent(ctrlc_win32_event());
+
 	while (!GetOverlappedResult(s, ovl, &result, FALSE)) {
 		DWORD r;