csi: implement "Send Device Attributes (Tertiary DA)"

csi.c

@@ -1248,6 +1248,24 @@ csi_dispatch(struct terminal *term, uint8_t final)
         break; /* private == '!' */
     }
 
+    case '=': {
+        switch (final) {
+        case 'c':
+            /* Send Device Attributes (Tertiary DA) */
+
+            /* Note: this is supposed to be the "unit ID". But it is
+             * unclear what the format *is*. Here I've simply copied
+             * the reply from xterm */
+            term_to_slave(term, "\033P!|00000000\033\\", 14);
+            break;
+
+        default:
+            UNHANDLED();
+            break;
+        }
+        break; /* private == '=' */
+    }
+
     default:
         UNHANDLED();
         break;