server(
getenv) ->path,err
Find the server's socket
getenv reads a variable, os.getenv by default. $PULSE_SERVER comes
first: its first entry that names a Unix socket, as unix:/path or as a
bare absolute path. Steam's container sets it to the host's socket, bound
inside. A {machine-id} prefix is dropped, since a socket path is local
to the machine anyway. With $PULSE_SERVER unset, the socket is
$XDG_RUNTIME_DIR/pulse/native.
Returns nil and an error when $PULSE_SERVER names no Unix socket, or
when neither variable is set. TCP is not taken.