* mh-customize.el (mh-index): Rename group to mh-search and sort group

* mh-customize.el (mh-index): Rename group to mh-search and sort group
definition and options accordingly.
(mh-index-program): Rename to mh-search-program.
(mh-kill-folder-suppress-prompt-hooks): Rename mh-index-p to
mh-search-p.
(mh-search-mode-hook): Change group from mh-index to mh-search.
(mh-index-folder): Rename to mh-search-folder. Change group from
mh-index to mh-search.

* mh-e.el (mh-folder-font-lock-keywords): Rename mh-index-folder to
mh-search-folder.

* mh-search.el (mh-indexer) Rename to mh-searcher. The commands pick
and grep are searchers too but aren't indexed.
(mh-index-execute-search-function): Rename to mh-search-function.
(mh-index-next-result-function): Rename to
mh-search-next-result-function.
(mh-index-regexp-builder): Rename to mh-search-regexp-builder.
(mh-search): Since redo-search-flag defaults to nil and is of lesser
importance, make it an optional argument and place it after the folder
and search-regexp arguments. Sync docstring with manual.
(mh-search-mode-map): Autoload so that keys are shown in help even
before mh-search is loaded.
(mh-search-mode): Sync docstring with manual.
(mh-index-do-search): Rename argument indexer to searcher. Sync
docstring with manual.
(mh-pick-do-search): Sync docstring with manual.
(mh-index-p): Rename to mh-search-p.
(mh-indexer-choices): Rename to mh-search-choices.
(mh-index-choose): Rename to mh-search-choose. Rename argument indexer
to searcher.
(mh-swish++-execute-search, mh-swish-execute-search)
(mh-mairix-execute-search, mh-namazu-execute-search): Drop "and read
the results" from docstring since these functions don't.
(mh-pick-execute-search, mh-grep-execute-search): Sync docstring with
manual.
(mh-index-generate-pretty-name): Prune -search from string so that
folder names for pick searches are the same as those of other
searches.