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.