(bibtex-sort-entry-class): Add safe-local-variable predicate.
(bibtex-sort-entry-class-alist): Don't set the global value.
(bibtex-init-sort-entry-class-alist): New fun.
(bibtex-sort-buffer, bibtex-prepare-new-entry): Call it to compute
bibtex-init-sort-entry-class-alist from the buffer-local value (if any)
of bibtex-init-sort-entry-class.