Add gnus-search-grep abstract engine

Add gnus-search-grep abstract engine

* lisp/gnus/gnus-search.el (gnus-search-grep): New abstract mixin
  engine, providing 'grep-program and 'grep-options slots.
  (gnus-search-grep-search): Method for doing secondary grep searches
  over previous search results.
  (gnus-search-find-grep): Inherit from gnus-search-grep.
  (gnus-search-indexed): Likewise.
  (gnus-search-indexed-parse-output): Add a grep pass pas part of this.
  (gnus-search-run-search): Use the grep options in the find-grep
  engine.
  (gnus-search-prepare-query): Find the grep: key when parsing the
  query.