event/SocketMonitor: add method Open()

Allow creating a closed SocketMonitor instance.

src/event/SocketMonitor.hxx

@@ -54,6 +54,9 @@ public:
 	static constexpr unsigned ERROR = G_IO_ERR;
 	static constexpr unsigned HANGUP = G_IO_HUP;
 
+	SocketMonitor(EventLoop &_loop)
+		:fd(-1), loop(_loop), source(nullptr) {}
+
 	SocketMonitor(int _fd, EventLoop &_loop);
 
 	~SocketMonitor();
@@ -68,6 +71,8 @@ public:
 		return fd;
 	}
 
+	void Open(int _fd);
+
 	void Close();
 
 	void Schedule(unsigned flags) {