hipchat: Fix access to /usr/share/X11/xkb

HipChat (or rather its copy of Qt) expects to find keyboard data in
/usr/share/X11/xkb. So use a LD_PRELOAD library to intercept and
rewrite the Glibc calls that access those paths. We've been doing the
same thing with packages like Spotify, but now this functionality has
been abstracted into a reusable library, libredirect.so. It uses an
environment variable $NIX_REDIRECTS containing a colon-separated list
of path prefixes to be rewritten, e.g. "/foo=bar:/xyzzy=/fnord".