VMS: Rethink the staging directory

On Windows and Unix, the staging directory $(DESTDIR) can simply be
prepended to the installation directory.  An attempt was made to do
something similar on VMS, but that ended up being a half measure
solution.  Instead of that, simply use the staging directory as a
prefix under which [.OPENSSL-INSTALL] and [.OPENSSL-COMMON] will hold
the two directory trees that should end up in the directories
indicated by --prefix and --openssldir, and finish the installation
with appropriate instructions on what to do next.

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