Pipes on VMS do not work well with binary data, use an intermediary file

VMS being a record oriented operating system, it's uncertain how the
'pipe' passes binary data from one process to another.  Experience
shows that we get in trouble, and it's probably due to the pipe in
itself being opened in text mode (variable length records).

It's safer to pass data via an intermediary file instead.

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