Emacs: Fix socket cleanup

[TLATER]

Emacs unconditionally cleans up its socket when it shuts down, but systemd expects the socket to be kept alive.

Hence, we need to teach emacs not to do that.

Seems there's a variable server--external-socket-initialized that will detect if systemd passed the socket to us.

There's also server-sentinel in server.el:396, which cleans up the socket file. Presumably all that needs to be done is to make it check whether an external process created the socket, and if it did, not clean it.