(bibtex-global-key-alist, bibtex-initialize): Exclude current buffer

(bibtex-global-key-alist, bibtex-initialize): Exclude current buffer
if it does not use bibtex-mode.
(bibtex-parse-strings, bibtex-strings, bibtex-initialize)
(bibtex-sort-buffer, bibtex-search-entry)
(bibtex-prepare-new-entry, bibtex-clean-entry): Use functionp to
test whether bibtex-strings and bibtex-reference-keys have their
initial values.