event/ServerSocket: pass UniqueSocketDescriptor by value

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

src/output/plugins/httpd/HttpdOutputPlugin.cxx

@@ -118,7 +118,7 @@ HttpdOutput::Unbind()
  * HttpdOutput.clients linked list.
  */
 inline void
-HttpdOutput::AddClient(UniqueSocketDescriptor &&fd)
+HttpdOutput::AddClient(UniqueSocketDescriptor fd)
 {
 	auto *client = new HttpdClient(*this, std::move(fd), GetEventLoop(),
 				       !encoder->ImplementsTag());
@@ -151,7 +151,7 @@ HttpdOutput::RunDeferred()
 }
 
 void
-HttpdOutput::OnAccept(UniqueSocketDescriptor &&fd,
+HttpdOutput::OnAccept(UniqueSocketDescriptor fd,
 		      SocketAddress address, gcc_unused int uid)
 {
 	/* the listener socket has become readable - a client has