bibtex-string-file-path and bibtex-file-path are lists of directories

bibtex-string-file-path and bibtex-file-path are lists of directories

* lisp/textmodes/bibtex.el (bibtex-string-file-path, bibtex-file-path):
These user options are now lists of directories.  For backward compatibility,
the old, obsolete format is still supported.
(bibtex-string-files-init, bibtex-initialize): Change accordingly.
Use file-name-extension and file-name-with-extension.
* etc/NEWS: Entry for new format of bibtex-string-file-path and
bibtex-file-path.

(cherry picked from commit 14b5ba7c61fd8fb1e2c8fe956ee925b4a59c10c8)