The only remaining difference is that in the system-xmlsec case we work
with the default key manager, not with the one that's only added by our
xmlsec patches.

This works for me for the uses I know of (see
<https://lists.freedesktop.org/archives/libreoffice/2017-February/076947.html>
for the motivation): signing and verifying of different signatures (bad
signature, good with non-trusted CA, good with trusted CA) with
software-based certificates all behave as expected.

Change-Id: If3f3e2b8373ab7397db3f98070a5a2ce51fa7c06
Reviewed-on: https://gerrit.libreoffice.org/39075Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-on: https://gerrit.libreoffice.org/39274Reviewed-by: Michael Stahl <mstahl@redhat.com>
Reviewed-by: Rene Engelhard <rene@debian.org>
Tested-by: Rene Engelhard <rene@debian.org>