Centralise chrootdir and cgroup prefix values.

They are used in all files that require them instead of hard coded
paths there. Exception is the dj_make_*chroot scripts, which do
not have the path but take it on the command line. We may want to
refactor those into a less duplicated solution.
Add judgedaemon init script.

Fixes #145
Move daemonization to after initial API communications.

During the initial setup the daemon will exit if e.g. communication fails.
Therefore, only daemonize just before we enter the main loop, which will
retry if there are failures. The init script will only report a
successful startup when initial communication is working.