fix off-by-one in dup2

[Christopher Faylor]

On Wed, Sep 25, 2013 at 05:26:25PM -0600, Eric Blake wrote:
>Solves the segfault here: http://cygwin.com/ml/cygwin/2013-09/msg00397.html
>but does not address the fact that we are still screwy with regards to
>rlimit.

Corinna reminded me about this.

Sorry for the delay in responding.  I was investigating if setdtablesize
should set an errno on error but it is difficult to say if it should
since it seems not to be a POSIX or Linux.  So, I guess we can just say
that it should set EINVAL.  Would you mind making that minor change and
checking this in?

cgf