Support functions for geard-idler

* Adds idler as a new subcommand of gear
* Adds new container events watcher which can be used to trigger actions based on systemd dbus events

Implementation notes:
* Adds a new flag file to keep track of idled containers. Eg: ```/var/lib/containers/units/00/0001.idle```
* Automatically tried to recover from invalid or stale IPTable rules
* Goes to file to look for container.target vs container-active.target since systemd is not reliable for that info.
* Looks at IPTable packet counts to monitor traffic
* Uses IPTables Netfilter queue to queue and start containers before letting packet continue