[feature request] Allow binding rpc to unix domain socket rather than a tcp port

Currently (as far as I can tell from the docs) if you want to host a local persistent aria2 daemon you have to have the daemon bind to a port, potentially with `--rpc-listen-all false` to limit it to only localhost. While that is good enough for the majority of cases, it still takes up a tcp port and hits more of the networking stack than it strictly should, plus it's harder to manage permissions on tcp ports than it is on unix domain sockets