lnd and lightning-loop resolve `localhost` to an IPv4 address when
creating RPC sockets.
Since NixOS 23.05, RTL (nodejs) resolves `localhost` to an IPv6
address when connecting to lnd and lightning-loop, which leads to
connection errors.
To fix these and other potential errors, replace all instances
of `localhost` with `127.0.0.1`.
Read uid/gid directly from the service pid.
This makes this fn work with arbitrary services, and with `bitcoind`,
where, for historical reasons, the service user name (`bitcoin`) doesn't
equal the service name.
