(bibtex-initialize): New autoloaded command. Rename from function

(bibtex-initialize): New autoloaded command. Rename from function
bibtex-files-expand. New optional arg select.
(bibtex-flash-head): Allow blink-matching-delay being zero.
(bibtex-clean-entry): Use atomic-change-group.
(bibtex-format-entry): Check presence of required fields only after
formatting of fields. Use member-ignore-case. Do not use
bibtex-parse-entry. Do not use booktitle field to set a missing
title.
(bibtex-autofill-entry): Do not call undo-boundary.
(bibtex-lessp): Handle crossref keys that point to another bibtex
file.
(bibtex-sort-buffer, bibtex-prepare-new-entry, bibtex-validate):
Parse keys if necessary.