bug#33847: 27.0.50; emacsclient does not find server socket

[Ulrich Mueller]

The master branch was recently updated to place the server socket in
XDG_RUNTIME_DIR, with a fallback to the previous TMPDIR location.
This will make emacsclient fail when the server has been started from
an environment where XDG_RUNTIME_DIR is not set.

For example, if emacs --daemon is started on a Gentoo system via
OpenRC's start-stop-daemon, then emacs will create the socket in
${TMPDIR}/emacs${UID}/, but emacsclient (in the user's X session)
will search for it in ${XDG_RUNTIME_DIR}/emacs/:

$ emacsclient -c
emacsclient: can't find socket; have you started the server?
emacsclient: To start the server in Emacs, type "M-x server-start".
emacsclient: No socket or alternate editor.  Please use:

        --socket-name
        --server-file      (or environment variable EMACS_SERVER_FILE)
        --alternate-editor (or environment variable ALTERNATE_EDITOR)

(The reason is of course that start-stop-daemon does not set the XDG_*
variables. However, I don't see how it could do that in any reasonable
way. Presumably it would have to happen via PAM and ConsoleKit, but the
latter doesn't have a display at that point.)

Suggested solutions:
- Create the socket in a dir that is more readily available, for example 
  somewhere under ${HOME}/emacs.d/, or
- Have emacsclient fall back to TMPDIR as well when no socket is found
  under XDG_RUNTIME_DIR.