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.

otherbackends/telnet.c

@@ -721,6 +721,7 @@ static char *telnet_init(const BackendVtable *vt, Seat *seat,
     int addressfamily;
 
     telnet = snew(Telnet);
+    memset(telnet, 0, sizeof(Telnet));
     telnet->plug.vt = &Telnet_plugvt;
     telnet->backend.vt = vt;
     telnet->conf = conf_copy(conf);