dcs: sort of implement DCS passthrough

We now store the passthrough characters in a buffer, and call
dcs_passthrough() on unhook.

However, dcs_passthrough() doesn't do anything.

terminal.h

@@ -129,6 +129,11 @@ struct vt {
         size_t size;
         size_t idx;
     } osc;
+    struct {
+        uint8_t *data;
+        size_t size;
+        size_t idx;
+    } dcs;
     struct {
         uint8_t data[4];
         size_t idx;