bootctl: use pager in help

The output is more than one page, let's enable the pager.

src/boot/bootctl.c

@@ -132,6 +132,8 @@ static int help(int argc, char *argv[], void *userdata) {
         _cleanup_free_ char *link = NULL;
         int r;
 
+        pager_open(arg_pager_flags);
+
         r = terminal_urlify_man("bootctl", "1", &link);
         if (r < 0)
                 return log_oom();