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/raw.c

@@ -156,6 +156,7 @@ static char *raw_init(const BackendVtable *vt, Seat *seat,
     char *loghost;
 
     raw = snew(Raw);
+    memset(raw, 0, sizeof(Raw));
     raw->plug.vt = &Raw_plugvt;
     raw->backend.vt = vt;
     raw->s = NULL;