The implementation is similar to the load/discard dance when a load
fails. New VGC functions are introduced iff the VCL has at least one
VMOD handling events.

The generated code looks like this:

	static unsigned vgc_inistep;
	static unsigned vgc_warmupstep;

	...

	static int
	VGC_Load(VRT_CTX)
	{
		...
	}

	static int
	VGC_Discard(VRT_CTX)
	{
		...
	}

	static int
	VGC_Warmup(VRT_CTX, enum vcl_event_e ev)
	{

		vgc_warmupstep = 0;

		/* 4 */
		if (Vmod_debug_Func._event(ctx, &vmod_priv_debug, ev))
			return (1);
		vgc_warmupstep = 4;

		return (0);
	}

	static int
	VGC_Use(VRT_CTX, enum vcl_event_e ev)
	{

		/* 4 */
		if (Vmod_debug_Func._event(ctx, &vmod_priv_debug, ev))
			return (1);

		return (0);
	}

	static int
	VGC_Cooldown(VRT_CTX, enum vcl_event_e ev)
	{
		int retval = 0;

		/* 4 */
		if (vgc_warmupstep >= 4 &&
		    Vmod_debug_Func._event(ctx, &vmod_priv_debug, ev) != 0)
			retval = 1;

		return (retval);
	}

	static int
	VGC_Event(VRT_CTX, enum vcl_event_e ev)
	{
		if (ev == VCL_EVENT_LOAD)
			return(VGC_Load(ctx));
		if (ev == VCL_EVENT_WARM)
			return(VGC_Warmup(ctx, ev));
		if (ev == VCL_EVENT_USE)
			return(VGC_Use(ctx, ev));
		if (ev == VCL_EVENT_COLD)
			return(VGC_Cooldown(ctx, ev));
		if (ev == VCL_EVENT_DISCARD)
			return(VGC_Discard(ctx));

		return (1);
	}

However, if there are no VMODs handling events, no new functions shall
be generated, leading to code looking like this:

	static unsigned vgc_inistep;
	static unsigned vgc_warmupstep;

	...

	static int
	VGC_Load(VRT_CTX)
	{
		...
	}

	static int
	VGC_Discard(VRT_CTX)
	{
		...
	}

	static int
	VGC_Event(VRT_CTX, enum vcl_event_e ev)
	{
		if (ev == VCL_EVENT_LOAD)
			return(VGC_Load(ctx));
		if (ev == VCL_EVENT_DISCARD)
			return(VGC_Discard(ctx));

		(void)vgc_warmupstep;
		return (0);
	}