return true for exactly the characters treated as whitespace by their flex
scanners. Per report from Victor Snezhko and subsequent investigation.
Also fix a passel of unsafe usages of <ctype.h> functions, that is, ye olde
char-vs-unsigned-char issue. I won't miss <ctype.h> when we are finally
able to stop using it.
remove the old isbn_issn module which is about to be obsoleted by EAN13.
contrib/isn is by Germán Méndez Bravo. Our thanks to Garrett A. Wollman
for having written the original isbn_issn module.
