Add --unix option for RPC server

[link2xt]

RPC server currently listens on stdin and prints to stdout. To support long-running server for cmsend and similar projects it needs to be able to accept more connections while running.

--unix option should accept a socket path, open it and listen for new connections. tokio::net::UnixListener is supported on all platforms (including Windows). yerpc should be able to handle this with multiple RpcSession objects opened per connection, if there are any problems we should fix it in yerpc as it is part of chatmail.

See also my comment at https://github.com/chatmail/core/pull/7535#discussion_r2573126848