WinHugs, interact support

Hi, This patch adds interact and getChar support to WinHugs. The implementation in this version is superior to that in the old version of WinHugs, including that the stop button now stops immediately (not after one char is pressed) and the status bar changes to advise the user that input is sought. The appropach uses a Content semaphore to pause the calculating thread when no input is available. Like all threading code, this is not particularly fun to write or easy to understand. Thanks Neil