According to http://msdn.microsoft.com/en-us/library/5814770t.aspx the "supported" errno values in VS2010 are

    E2BIG EACCES EAGAIN EBADF ECHILD EDEADLOCK EDOM EEXIST EILSEQ
    EINVAL EMFILE ENOENT ENOEXEC ENOMEM ENOSPC ERANGE EXDEV STRUNCATE

of which the ones with WSA equivalents are

    EACCES EBADF EINVAL EMFILE

The other constants listed in errno.h are for "compatibility with POSIX".

I take this to mean that the only errno values which will be set by libc are on the supported list.  We should not change any of these, but I think we can redefine the others to WSA equivalents without a problem.