Tweaks to the regexp apropos logic

* lib/lisp/swank-backend.lisp (make-cl-ppcre-matcher)
(make-plain-matcher): Removed.
(make-apropos-matcher): Tweak docstring. No default
implementation.

* lib/lisp/swank.lisp (make-cl-ppcre-matcher)
(make-plain-matcher): Moved here from swank-backend.lisp
(*try-cl-ppcre-for-apropos*): New parameter.
(apropos-symbols): Control apropos matcher to use here.
More apropos adjustments

* lib/lisp/swank.lisp (make-plain-matcher): Return search result
indexes.
(apropos-symbols): Improve CL-PPCRE fallback logic.

* lib/lisp/backend/swank-allegro.lisp (describe-symbol-for-emacs):
Ignore errors when calling DOCUMENTATION.
* sly.el (sly-display-eval-result): Use `message'

Also remove mention of sly-selector.
sly-autodoc is less noisy when there are no connections

* contrib/sly-autodoc.el (sly-autodoc): Check for live connection.

* contrib/sly-mrepl.el (sly-mrepl-inside-string-or-comment-p): Check for
live mrepl process.