Revert c1423a8 and fix things up

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.
Quiet a warning in test_plugin.c
Fix master (lib/krb5 warnings)
Make lib/hdb/hdb.c build (but hdb plugins broken)