Support multiple concurrent instances of Emacs + GDS server

Support multiple concurrent instances of Emacs + GDS server

By:

- Making the Unix socket name unique (for each Emacs instance), by
  appending Emacs's PID to it.

- Changing the GDS server to listen on both Unix domain and TCP (and
  not to mind if the TCP bind fails, which will happen if another GDS
  instance has already bound to the TCP port number).

- Adding this unique Unix socket name to the environment (as
  GDS_UNIX_SOCKET_NAME), so that Guile clients started from inside
  Emacs can pick it up.

- Changing the GDS client code to look for GDS_UNIX_SOCKET_NAME in the
  environment, and to connect to the Unix socket with that name
  instead of over TCP.

  Guile clients started outside Emacs will not find
  GDS_UNIX_SOCKET_NAME and so will fall back to using TCP.  This means
  they will connect to whichever Emacs + GDS server instance started
  first.

* emacs/gds-server.el (gds-start-server): Take both Unix socket name
  and TCP port args, instead of just one (which could be either Unix
  or TCP), and pass these on to `run-server'.  Remove unused optional
  bufname arg.

* emacs/gds.el (gds-unix-socket-name, gds-tcp-port): New variables.
  (gds-socket-type-alist): Removed.
  (gds-run-debug-server): Pass gds-unix-socket-name and gds-tcp-port
  to gds-start-server.  Add the Unix socket name to the environment.
  (gds-server-socket-type): Note now obsolete.

* ice-9/gds-client.scm (connect-to-gds): Get Unix socket name from
  environment, and connect to this in preference to using TCP.

* ice-9/gds-server.scm (run-server): Take both Unix socket name and
  TCP port args.  Listen and accept connections on both.