only allow dynamic UnregisterHandler for external handlers, thereby fixing DoS

Trying to unregister an internal handler ended up in a SEGFAULT, because
the tcmur_handler->opaque was NULL. Way to reproduce:

dbus-send --system --print-reply --dest=org.kernel.TCMUService1 /org/kernel/TCMUService1/HandlerManager1 org.kernel.TCMUService1.HandlerManager1.UnregisterHandler string:qcow

we use a newly introduced boolean in struct tcmur_handler for keeping
track of external handlers. As suggested by mikechristie adjusting the
public data structure is acceptable.