zeroconf/AvahiPoll: the struct timeval is an absolute time point

Fixes broken libavahi-client timeouts.

src/zeroconf/AvahiPoll.cxx

@@ -105,8 +105,28 @@ public:
 	}
 
 private:
+	[[gnu::pure]]
+	Event::Duration AbsoluteToDuration(const struct timeval &tv) noexcept {
+		if (tv.tv_sec == 0)
+			/* schedule immediately */
+			return {};
+
+		struct timeval now;
+		if (gettimeofday(&now, nullptr) < 0)
+			/* shouldn't ever fail, but if it does, do
+			   something reasonable */
+			return std::chrono::seconds(1);
+
+		auto d = ToSteadyClockDuration(tv)
+			- ToSteadyClockDuration(now);
+		if (d.count() < 0)
+			return {};
+
+		return d;
+	}
+
 	void Schedule(const struct timeval &tv) noexcept {
-		event.Schedule(ToSteadyClockDuration(tv));
+		event.Schedule(AbsoluteToDuration(tv));
 	}
 
 	void OnTimeout() noexcept {