Write our own chroot support using syscall instead of calling chroot binary

[joebonrichie]

Instead of calling out the to the chroot binary write our own chroot support using the syscall.

This should be able to handle stdin and spawn a bash binary.

The main benefit of this is that we can immediately chroot into the $workdir directory instead of /home/user/.

See f658849 as a starting point, the difference being it'll have to spawn bash and handle stdin.

[ermo]

Fixed in #107