Closes #5: don't ask irrelevant questions on sly-quit

Also don't on `sly-disconnect'

* sly.el (sly-auto-select-connection): Use `sly-prompt-for-connection'
(sly-connection-close-hook): Tweak message.
(sly-prompt-for-connection, sly-auto-select-connection)
(sly-select-connection, sly-cycle-connections-hook)
(sly-cycle-connections): Move below, keep unchanged.
(sly-prompt-for-connection): New function.
(sly-quit-lisp): Use `sly-prompt-for-connection'
(sly-eval-macroexpand-inplace): Fix byte-compiler warning.