darcs patch: More accurate isatty test for MinGW.

[Edward Z. Yang]

In particular, this patch fixes a bunch of known and unknown failing tests
on MinGW for GHC.  For a little background, _isatty on Windows doesn't
actually test for a TTY: it just tests for a character device [1].  So
it will spuriously report true for NUL.

[1] http://msdn.microsoft.com/en-us/library/f4s0ddew(v=VS.80).aspx

Excerpts from Edward Z. Yang's message of Tue Sep 07 15:55:40 -0400 2010:
> 1 patch for repository http://darcs.haskell.org/packages/base:
> 
> Tue Sep  7 11:41:44 EDT 2010  Edward Z. Yang <ezyang at mit.edu>
>   * More accurate isatty test for MinGW.