script protocol over unix socket

[slingamn]

auth-script and ip-check-script require a semaphore acquisition because executing an external process creates a scalability bottleneck.

We could run the same protocol (JSON in, JSON out) over unix domain socket and then it would be scalable enough not to need a semaphore. This would also provide advantages in the case where the source of truth works better over a persistent connection: consider a separate daemon that federates with Anope or Atheme, then exposes their password checking to Ergo over unix domain socket.