Fixed autorestart mode

This pull request carries the following changes:

* Fixed autorestart mode not working (b3.parser.exitcode was not being set and thus B3 exit code was always set to 0 unless an exception was being raised)
* Improved shell initialization script so it can live together with in-game commands for shutdown/reboot B3
* Let the parser know when B3 is being run in auto-restart mode (used an extra argument in argparse, hidden in the help text)
* Let the updater plugin reboot B3 (instead of shutdown) after a successfull update install (if B3 is running in auto-restart mode)
* Unregister !restart command in admin plugin if B3 is not running in auto-restart mode

Fixes issue described in the last 3 comments of #234