Add memsets after allocation of all Backend implementors.

Now every struct that implements the Backend trait is completely
cleared before we start initialising any of its fields. This will mean
I can add new fields that default to 0 or NULL, without having to mess
around initialising them explicitly everywhere.

unix/pty.c

@@ -414,6 +414,7 @@ static void pty_open_master(Pty *pty)
 static Pty *new_pty_struct(void)
 {
     Pty *pty = snew(Pty);
+    memset(pty, 0, sizeof(Pty));
     pty->conf = NULL;
     pty->pending_eof = false;
     bufchain_init(&pty->output_data);

unix/serial.c

@@ -294,6 +294,7 @@ static char *serial_init(const BackendVtable *vt, Seat *seat,
     seat_set_trust_status(seat, false);
 
     serial = snew(Serial);
+    memset(serial, 0, sizeof(Serial));
     serial->backend.vt = vt;
     *backend_handle = &serial->backend;