conf: log error when encountering multiple filterrefs in interface

conf: log error when encountering multiple filterrefs in interface

Each interface has a single pointer to a filterref object. That
filterref can itself point to multiple other filterrefs, but at the
toplevel there is only one.

The parser had previously just silently overwritten earlier filterrefs
when a new one was encountered, so the interface was left with
whichever was the last filterref in the xml, ignoring all the
others. This patch logs an error when it sees more than one filterref.