Single user mode / maintenance mode

Operations such as working with innodb_force_recovery or applying binlogs on a server after recovering with backups, are usually done with skip-networking enabled to avoid having clients run potentially harmful writes (or that would lead to crashes with force recovery), but even when not accepting TCP connections the server would still be open for local connections and having cronjobs connecting through localhost is pretty common, so having a single-user or maintenance mode would come in pretty handy (of course, one can start the server with a different port, but seems like a poor hack and could not be done on-line)