Creates the socket's parent directory and checks it is safe to use.

Directories this call creates are owner-only: the /tmp/obsctl-$UID/ fallback sits in a world-writable parent, and one left at the default umask would let any local user replace the socket file even though the socket itself is 0600.

A directory that already exists is left alone. It may be a deliberate choice — server.socket_path: /tmp/obsctl.sock puts the socket straight into /tmp — and silently chmod'ing a shared directory would be a worse surprise than the problem it solves.