Create socket in /var/run/user/${HOME}

Use the runtime dir from XDG Base Directory Specification for creating the
socket in a directory only the user can read and write.
diff --git a/clients/window.c b/clients/window.c
index c77a8ce..d03ef4b 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -1337,8 +1337,6 @@
 	}
 }
 
-static const char socket_name[] = "\0wayland";
-
 static void
 display_render_frame(struct display *d)
 {
@@ -1425,7 +1423,7 @@
 	if (d == NULL)
 		return NULL;
 
-	d->display = wl_display_connect(socket_name, sizeof socket_name);
+	d->display = wl_display_connect(NULL);
 	if (d->display == NULL) {
 		fprintf(stderr, "failed to create display: %m\n");
 		return NULL;