Use dpkg-divert to move /usr/bin/ischroot

dpkg-divert is [the correct way](http://www.debian-administration.org/article/118/Replacing_binaries_with_dpkg-divert) to move around binaries belonging to a package on Debian-flavored systems. Updates will respect diversions and will not overwrite our fake ischroot with an updated version, whereas by simply moving the file there is nothing to prevent this.