Improvements to sly-mrepl teardown logic

* sly.el (sly-net-close): Unset sentinel before killing process
explicitly.

* contrib/sly-mrepl.el (sly-mrepl): Use `sly-mrepl-teardown-repls'.
(sly-mrepl-teardown-repls): Renamed from `sly-mrepl-close-repls'. Call
`sly-mrepl--teardown'.
(sly-mrepl--ensure-newline): Check for bobp
(sly-mrepl-return): Improve assertions and messages.
(sly-mrepl--teardown): Simplify and ignore `sly-connection' errors.
Improvements to apropos

New interface allows lisp implementations to use regexp
matching. Consistently display package qualifier in results unless
under special conditions.

* sly.el (sly-apropos-all): Better prompt.
(sly-apropos): Improve prompt. Don't use `sly-current-package'.

* lib/lisp/swank-backend.lisp (make-apropos-matcher): New interface.

* lib/lisp/swank.lisp (apropos-list-for-emacs): Set
*BUFFER-PACKAGE* criteriously.
(make-apropos-matcher): Removed.
(apropos-symbols): Use new interface.