hookd: restrict and authenticate the hook socket #282

The hook socket (<root>/hook.sock, internal/hookd/hookd.go, Serve) trusts the repository id, user id and key scope in each request. Any process that can connect can ask for a push decision as any user and trigger post-receive side effects as them. Its only protection is the data directory's permissions; the socket itself is created with the process umask and no explicit mode.

• chmod 0600 the socket after Listen.
• Check the peer's uid with SO_PEERCRED (Linux) and refuse other users.
• Consider a per-push token passed to the hook in its environment, so a request must name a receive-pack the daemon started.