Add textsec support for confusable characters

Add textsec support for confusable characters

* admin/notes/unicode: Note the confusables.txt file.
* admin/unidata/Makefile.in (${unidir}/uni-confusable.el):
Generate the confusable file.

* admin/unidata/README (https): Add confusables.txt.

* admin/unidata/confusables.txt: New file.

* admin/unidata/unidata-gen.el (unidata-gen-confusable): Parse the
confusables.txt file.

* lisp/international/textsec.el (textsec-ascii-confusable-p)
(textsec-unconfuse-string): New functions.