thread/Cond: add wait() overload which takes a unique_lock<>

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

src/neighbor/plugins/SmbclientNeighborPlugin.cxx

@@ -238,7 +238,7 @@ SmbclientNeighborExplorer::ThreadFunc() noexcept
 {
 	SetThreadName("smbclient");
 
-	const std::lock_guard<Mutex> lock(mutex);
+	std::unique_lock<Mutex> lock(mutex);
 
 	while (!quit) {
 		Run();
@@ -247,7 +247,7 @@ SmbclientNeighborExplorer::ThreadFunc() noexcept
 			break;
 
 		// TODO: sleep for how long?
-		cond.wait_for(mutex, std::chrono::seconds(10));
+		cond.wait_for(lock, std::chrono::seconds(10));
 	}
 }