As previously advertised, remove the old libassuan port now that all
consumers have moved to libassuan 2.0.0, or dropped the dependency

Update security/libassuan to version 2.0.0, which is required by gnupg 2.0.15

Unfortunately version 2.0.0 is largely incompatible with version 1.x, so it
is necessary to have a stopgap measure while ports that depend on libassuan
can be updated. In conversation with the maintainers of the dependent ports
it was originally considered ideal to prepare updates for the ports first,
then upgrade everything to libassuan 2.x en masse. Since no action has
arisen on that front, go with plan B:

Copy security/libassuan to security/libassuan-1, and update the dependent
ports accordingly. Because this is (intended to be) a _temporary_ measure,
and because no updates for libassuan 1.x are anticipated, and because the
hope is that it can be removed sooner rather than later, it's a copy instead
of a repocopy.