(elmo-make-search-engine): Use prefix

(elmo-make-search-engine): Use prefix
`elmo-search-' for require symbol instead of `else-'.
(elmo-search-engine-extprog): New class.
(elmo-search-engine-do-search): Remove nil from argument
list. Added argument pattern to evaluate element as function.
(elmo-search-engine-local-file): Derived from
`elmo-search-engine-extprog'.
(elmo-search-engine-create-message-entity): Check existance of file.
(elmo-search-engine-fetch-message): Ditto.
(elmo-search-namazu-index): Follow the API change.
(elmo-search-grep-target): Ditto.