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/supdup.c

@@ -683,6 +683,7 @@ static char *supdup_init(const BackendVtable *x, Seat *seat,
     const char *utf8 = "\033%G";
 
     supdup = snew(struct supdup_tag);
+    memset(supdup, 0, sizeof(Supdup));
     supdup->plug.vt = &fn_table;
     supdup->backend.vt = &supdup_backend;
     supdup->logctx = logctx;