failed query to a `forward only` forwarder increments `serverquota` counter (spilled due to server quota)

As observed in Support ticket #16297

I was inspecting the stats output and was very surprised to see this:

13779 spilled due to server quota

The server in question does not have fetches-per-server configured, so this defaults to zero (unlimited). But yet...

Looking at the code - I suspect there's a failure mode that drops through the 'out' block in fctx_getaddresses() without resetting all_spilled (which starts at 'true').

static isc_result_t
fctx_getaddresses(fetchctx_t *fctx, bool badcache) {
	dns_rdata_t rdata = DNS_RDATA_INIT;
	isc_result_t result;
	dns_resolver_t *res;
	isc_stdtime_t now;
	unsigned int stdoptions = 0;
	dns_forwarder_t *fwd;
	dns_adbaddrinfo_t *ai;
	bool all_bad;
	dns_rdata_ns_t ns;
	bool need_alternate = false;
	bool all_spilled = true;

...

			/*
			 * If all of the addresses found were over the
			 * fetches-per-server quota, return the configured
			 * response.
			 */
			if (all_spilled) {
				result = res->quotaresp[dns_quotatype_server];
				inc_stats(res, dns_resstatscounter_serverquota);
			}

This is a server that is using global forwarding, so we skip case 'normal_nses', which is where 'all_spilled' is normally reset from true to false during processing:

	if (fctx->fwdpolicy == dns_fwdpolicy_only)
		goto out;

So I'm guessing that what's been 'counted' and then reported here, is failures in getting responses back from any of the global forwarders (which tallies quite nicely with the problem I'm investigating - even though this wasn't a counter I was expecting to see in the stats!).

The assumption seems to be if it's a failure for any other reason than fetch-limits, that something will reset the 'all_spilled' flag - it would appear that assumption is flawed for some configurations and situations. Could someone have a look at this please - it should be an easy one to fix.

I note that this has also been noticed before on bind-users:

https://lists.isc.org/pipermail/bind-users/2016-June/097011.html

I observed this in 9.11.15-S1, but the code path looks the same still on master.

Requested changes:

• fix serverquota counter
• add a new counter for specifically for situation when all forwarders have failed