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.

lib/krb5/krb5.h

@@ -869,6 +869,8 @@ enum krb5_plugin_type {
     PLUGIN_TYPE_FUNC
 };
 
+#define KRB5_PLUGIN_INVOKE_ALL  1
+
 struct credentials; /* this is to keep the compiler happy */
 struct getargs;
 struct sockaddr;