Refactor parsing of indexed search engine output

Refactor parsing of indexed search engine output

* lisp/gnus/gnus-search.el (gnus-search-indexed-parse-output): Rename
  `gnus-search-indexed-massage-output' to this. All indexed search
  engines now use this method.
  (gnus-search-index-extract): This new method is now distinct to each
  engine. All it does is extract a single search result from the
  output buffer.

Remove `gnus-search-add-result' and `gnus-search-compose-result',
these are now part of `gnus-search-indexed-parse-output'.