VT100 escape sequences appearing on Windows 10 Console - #3897

During opam init, things proceed nicely until a second or so after the repository tarball is downloaded when the "Processing" message suddenly appears as:

<><> Fetching repository information ><><><><><><><><><><><><><><><><><><><><><>
�[K[�[32mdefault�[0m] Initialisedlt�[0m: http]
�[KProcessing  1/1:
�[36m<><>�[0m �[01mRequired setup - please read�[0m
�[36m<><><><><><><><><><><><><><><><><><><><><><><>�[0m   

The problem is that processes invoked by opam gain stdin and therefore the Console and clear the Console mode (in particular, Cygwin processes seem to do this). They're running in the background, so it doesn't matter whether they behave nicely and restore the console mode on termination, because the message may well be displayed while they're running.

This PR offers two "fixes":

• firstly the OpamConsole mode is altered to force-set the mode on every print (this is ugly, but not much more ugly than what was there before)
• secondly the default for allow_stdin is declared false on Windows, and a couple of places where allow_stdin should clearly be false (cygpath invocation and the download command) are set

Either of these on their own is sufficient to fix the Windows problem.