Problem: Client blocks on destroy if no messages pending on dealer

Solution: return -1 to interrupt zloop on $TERM

@hintjens 
Please look this over and see if I jusst missed some other mechanism already in place.  The code where this fails is actually the client test generated by zproto:
```c
    //  @selftest
    zactor_t *client = zactor_new (myclientclass, NULL);
    if (verbose)
        zstr_send (client, "VERBOSE");
    zactor_destroy (&client); // this line blocks forever
    //  @end
```