Centralized thread cancellation in processor_t

This ensures that no threads are active when plugins and the rest of the
daemon are unloaded.

callback_job_t was simplified a lot in the process as its main
functionality is now contained in processor_t.  The parent-child
relationships were abandoned as these were only needed to simplify job
cancellation.

src/libcharon/plugins/dhcp/dhcp_socket.c

@@ -105,11 +105,6 @@ struct private_dhcp_socket_t {
 	 * DHCP server address, or broadcast
 	 */
 	host_t *dst;
-
-	/**
-	 * Callback job receiving DHCP responses
-	 */
-	callback_job_t *job;
 };
 
 /**
@@ -613,10 +608,6 @@ static job_requeue_t receive_dhcp(private_dhcp_socket_t *this)
 METHOD(dhcp_socket_t, destroy, void,
 	private_dhcp_socket_t *this)
 {
-	if (this->job)
-	{
-		this->job->cancel(this->job);
-	}
 	while (this->waiting)
 	{
 		this->condvar->signal(this->condvar);
@@ -761,9 +752,9 @@ dhcp_socket_t *dhcp_socket_create()
 		return NULL;
 	}
 
-	this->job = callback_job_create_with_prio((callback_job_cb_t)receive_dhcp,
-									this, NULL, NULL, JOB_PRIO_CRITICAL);
-	lib->processor->queue_job(lib->processor, (job_t*)this->job);
+	lib->processor->queue_job(lib->processor,
+		(job_t*)callback_job_create_with_prio((callback_job_cb_t)receive_dhcp,
+			this, NULL, (callback_job_cancel_t)return_false, JOB_PRIO_CRITICAL));
 
 	return &this->public;
 }