Explain teardown reason in sly-mrepl

* sly.el (sly-maybe-start-lisp, sly-net-close, sly-net-sentinel)
(sly-net-read-or-lose, sly-check-version, sly-disconnect)
(sly-disconnect-all, sly-restart-sentinel, sly-quit-sentinel):
Pass close reason to `sly-net-close'

* contrib/sly-mrepl.el (sly-mrepl--teardown-repls): fetch teardown
reason from process args.
(sly-mrepl--teardown): Take new optional reason arg.
Fix read-only and field navigation issues in sly-mrepl

* contrib/sly-mrepl.el (sly-mrepl--insert-output): Insert newline
explicitly, don't use `sly-mrepl--ensure-newline'. Solve read-line
issues.
(sly-mrepl--ensure-newline): Simplify implementation.
(:prompt, sly-mrepl--insert-returned-values): Don't pass mark to
`sly-mrepl--ensure-newline'