Provide more flexibility for definition of BibTeX entries.

Provide more flexibility for definition of BibTeX entries.

* lisp/textmodes/bibtex.el (bibtex-aux-opt-alist): Rename from
bibtex-user-optional-fields.
(bibtex-BibTeX-entry-alist, bibtex-biblatex-entry-alist): Permit
aliases that inherit their definition from another entry.
(bibtex-BibTeX-aux-entry-alist, bibtex-biblatex-aux-entry-alist)
(bibtex-BibTeX-aux-opt-alist, bibtex-biblatex-aux-opt-alist): New
user variables.
(bibtex-entry-alist): Honor these new variables.
(bibtex-field-alist): New function.
(bibtex-include-OPTkey): Default changed to nil, declared
obsolete.  Use bibtex-aux-opt-alist instead.
(bibtex-set-dialect): Use cl-flet and bibtex-field-alist.
(bibtex-dialect-select-map): New variable.
(bibtex-init-dialect): New function.
(bibtex-field-list): Simplify.
(bibtex-print-help-message): Bug fix.

(cherry picked from commit b26418694e8a573cd500d71029c0e8f53598ea3c)