VMS open() doesn't take O_BINARY, but takes a context description

VMS open() doesn't take O_BINARY, but takes a context description

Tell open() O_BINARY on VMS doesn't make sense, as it's possible to
use more precise file attributes.  However, if we're still going to
fdopen() it in binary mode, we must set the fd in binary context.

Reviewed-by: Rich Salz <rsalz@openssl.org>