xkb: Handle xkb formated string output safely (CVE-2017-13723)

Generating strings for XKB data used a single shared static buffer,
which offered several opportunities for errors. Use a ring of
resizable buffers instead, to avoid problems when strings end up
longer than anticipated.

Reviewed-by: Michal Srb <msrb@suse.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>
(cherry picked from commit 94f11ca5)