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/compositor/compositor-wayland.c b/compositor/compositor-wayland.c
index 98c675b..e289a29 100644
--- a/compositor/compositor-wayland.c
+++ b/compositor/compositor-wayland.c
@@ -513,8 +513,6 @@
 	struct wayland_compositor *c;
 	struct wl_event_loop *loop;
 	int fd;
-	char *socket_name, *env;
-	int socket_name_size;
 
 	c = malloc(sizeof *c);
 	if (c == NULL)
@@ -522,11 +520,7 @@
 
 	memset(c, 0, sizeof *c);
 
-	env = getenv("WAYLAND_DISPLAY");
-	socket_name_size = asprintf(&socket_name, "%c%s", '\0', env) + 1;
-
-	c->parent.display = wl_display_connect(socket_name, socket_name_size);
-	free(socket_name);
+	c->parent.display = wl_display_connect(NULL);
 
 	if (c->parent.display == NULL) {
 		fprintf(stderr, "failed to create display: %m\n");