Some updates for the MIT-Scheme backend.

* swank-mit-scheme.scm (accept-connections): Use the built-in
open-tcp-server-socket.  In recent releases it seems to set
SO_REUSEADDR and lately we used a hardcoded port number even with
netcat.
(swank:connection-info): Include the :encoding for create-repl.
(swank-repl:listener-eval): Renamed from swank:listener-eval.
(swank-repl:create-repl): Renamed from swank:create-repl.
Return the name of the nearest repl env.
(hack-quotes): New helper.
(emacs-rex): Use it, as swank-repl:create-repl is called with a
keyword argument.