agent: Respect non-standard --prefix when looking for packages

When built with a non-standard --prefix=/xxxx then override the
$XDG_DATA_DIRS environment variable in cockpit-agent so that
we find our packages in the installed location.

Fixes #1342
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
configure: Add --enable-prefix-only option to restrict to prefix

Restrict 'make install' to --prefix when '--enable-prefix-only'
is used. This will result in a cockpit that doesn't work without
further tweaking, but is an advanced tool for advanced use cases.

Fixes #1343
Closes #1346
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>