Also, we were stopping as soon as one registered plugin returned
something other than KRB5_PLUGIN_NO_HANDLE, but we weren't doing the
same for discovered plugins. Add KRB5_PLUGIN_INVOKE_ALL flag to deal
with this; by default we'll stop at the first plugin that returns
anything other than KRB5_PLUGIN_NO_HANDLE.
Other buglets fixed as in c1423a8.