Add commands 'kill-matching-lines' and 'copy-matching-lines'

Add commands 'kill-matching-lines' and 'copy-matching-lines'

* doc/emacs/search.texi: Document these additions.
* lisp/replace.el:
Add the commands 'kill-matching-lines' and 'copy-matching-lines'.

'kill-matching-lines' is like 'flush-lines', but adds the lines to the
kill ring as a single string, keeping line endings.
'copy-matching-lines' is like 'kill-matching-lines', but only copies
those lines instead of killing them.