compositor: Always set WAYLAND_DISPLAY

We used to only set it if we were given an argument on the command line.
It's useful to always set it so nested weston works out of the box.

src/compositor.c

@@ -2994,7 +2994,7 @@ int main(int argc, char *argv[])
 	int32_t idle_time = 300;
 	int32_t xserver = 0;
 	int32_t help = 0;
-	char *socket_name = NULL;
+	char *socket_name = "wayland-0";
 	char *config_file;
 
 	const struct config_key shell_config_keys[] = {
@@ -3088,6 +3088,8 @@ int main(int argc, char *argv[])
 	ec->option_idle_time = idle_time;
 	ec->idle_time = idle_time;
 
+	setenv("WAYLAND_DISPLAY", socket_name, 1);
+
 	module_init = NULL;
 	if (xserver)
 		module_init = load_module("xwayland.so", "module_init");
@@ -3096,9 +3098,6 @@ int main(int argc, char *argv[])
 		goto out;
 	}
 
-	if (socket_name)
-		setenv("WAYLAND_DISPLAY", socket_name, 1);
-
 	if (!shell)
 		shell = "desktop-shell.so";
 	module_init = load_module(shell, "module_init");