*: add "noexcept" to many, many function prototypes

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

src/output/plugins/OpenALOutputPlugin.cxx

@@ -58,7 +58,7 @@ class OpenALOutput {
 	void Close();
 
 	gcc_pure
-	std::chrono::steady_clock::duration Delay() const {
+	std::chrono::steady_clock::duration Delay() const noexcept {
 		return filled < NUM_BUFFERS || HasProcessed()
 			? std::chrono::steady_clock::duration::zero()
 			/* we don't know exactly how long we must wait