init: Adding respawn

As per issue #138, it would be very useful being able to restart the
deploiement process without rebooting.

The required steps to make it done properly:
- umounting filesystems
- killing some process
- removing temporary files

The temporary files are targetting log files but also files generated
during the boot process like those coming from detect.py, the configure
script or some files generated by /vars

The process to be killed are those locking the filesystems like udev,
dhclient etc..

The filesystem to be umounted are those from /chroot and all virtual
filesystem like /proc,/sys,/run. /dev could not be umounted as we run
respawn from a pts/ which locks /dev/pts/.

To trigger a respawn, just call /respawn