(bibtex-map-entries): Use marker to keep

(bibtex-map-entries): Use marker to keep
track of the buffer position of the end of a BibTeX entry as this
position may change during reformatting.
(bibtex-format-entry): Remove whitespace before processing
numerical fields so that we recognize the latter properly.
(bibtex-reformat): Do not use push which changes the global value
of bibtex-entry-format.
(bibtex-field-braces-alist, bibtex-field-strings-alist)
(bibtex-field-re-init): Replace only space characters by regexp
for whitespace.
(bibtex-generate-url-list, bibtex-cite-matcher-alist): Fix
docstring.
(bibtex-initialize): Also update bibtex-strings.
(bibtex-kill-field): Preserve white space at end of entry.
(bibtex-kill-entry, bibtex-yank-pop, bibtex-insert-kill): Update
bibtex-reference-keys.