Refresh the thinking of --prefix and --openssldir

--prefix is now exclusively used for software and manual installation.
--openssldir is not exclusively used as a default location for certs,
keys and the default openssl.cnf.

This change is made to bring clarity, to have the two less
intertwined, and to be more compatible with the usual ways of software
installation.

Please change your habits and scripts to use --prefix rather than
--openssldir for installation location now.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>