Allow checking for duplicate bindings with -C

- Having both parse_configuration and parse_file is excessive now
- We detect if we are parsing only by checking if conn is NULL, not with
use_nagbar
- font.pattern needs to be set to NULL because it is freed in
free_font()

Fixes #3660

src/commands.c

@@ -1573,7 +1573,7 @@ void cmd_reload(I3_CMD) {
     LOG("reloading\n");
     kill_nagbar(&config_error_nagbar_pid, false);
     kill_nagbar(&command_error_nagbar_pid, false);
-    load_configuration(NULL, true);
+    load_configuration(NULL, C_RELOAD);
     x_set_i3_atoms();
     /* Send an IPC event just in case the ws names have changed */
     ipc_send_workspace_event("reload", NULL, NULL);