fleetctl: service uptime for list-units

[dongsupark]

Allow fleetctl list-units to retrieve per-unit uptime from systemd, for example:

localhost # ./fleet/fleetctl list-units
UNIT MACHINE ACTIVE SUB UPTIME
world.service 06ecd4f7.../192.168.122.30 active running 2015-07-06 07:38:38 AM UTC, Since 11m39s
world2.service 1d3430ef.../192.168.122.31 active running 2015-07-06 07:48:24 AM UTC, Since 1m54s
world_glob.service 06ecd4f7.../192.168.122.30 active running 2015-07-06 07:48:00 AM UTC, Since 2m18s
world_glob.service 1d3430ef.../192.168.122.31 active running 2015-07-06 07:47:59 AM UTC, Since 2m18s

What's changed since #1293:

• Avoided unnecessary iteration for each unit when fetching systemd properties. Instead make use of setting internal UnitState, both for the normal loop for systemd (>= 230) and for the fallback loop for systemd (<= 229).
• Fixed typos and bugs in registry and schema
• Excluded ActiveEnterTimestamp from unit comparison condition.
• Added timezone e.g. UTC to the output

Fixes https://github.com/coreos/fleet/issues/1128 Supersedes https://github.com/coreos/fleet/pull/1293 /cc @wuqixuan

[jonboulle]

I'm not sure about this one - please hold off until I can give it a little more thought

[dongsupark]

@jonboulle I see.