Support automatically updating index.

Support automatically updating index.
(gnus-namazu-default-index-directory): New constant.
(gnus-namazu-make-index-interval, gnus-namazu-make-index-command,
gnus-namazu-make-index-arguments): New options.
(gnus-namazu/setup): Call `gnus-namazu-make-index'.
(gnus-namazu/real-group-name): Renamed from
`gnus-namazu/check-cache-group'.
(gnus-namazu/cache-group-candidates): Renamed from
`gnus-namazu/cache-group-candidates'.
(gnus-namazu/search): Experimental support of articles covered by
agent.
(gnus-namazu/default-index-directory, gnus-namazu/lapse-seconds,
gnus-namazu/mknmz-sentinel): New internal functions.
(gnus-namazu/mknmz-process): New internal variable.
(gnus-namazu/lock-file-name, gnus-namazu/index-file-name): New
macros.
(gnus-namazu-make-index, gnus-namazu-make-index-stop): New
commands.