Expose target machine/state fields in list-units

Add new `TSTATE` and `TMACHINE` fields to list-units, dropping some irrelevant states from the default list.

In the event that you have two units that conflict with one another, this was the old output you would see:

```
$ fleetctl list-units
UNIT		STATE	LOAD	ACTIVE		SUB	DESC	MACHINE
bar.service	inactive	-	-		-	-	-
foo.service	loaded	loaded	inactive	dead	-	5849e100.../172.17.8.101
```

...and the new output:

```
$ fleetctl list-units
UNIT		TSTATE		TMACHINE			STATE		MACHINE				ACTIVE
bar.service	launched	-				inactive	-				-
foo.service	loaded		5849e100.../172.17.8.101	loaded		5849e100.../172.17.8.101	inactive
```

This new format at least gives you an idea of what bar.service should be doing
Fair point. Until we fix #638, I believe it is incredibly important to expose in the default output. Once we feel more confident in how we're publishing state, I'm fine with removing it from the default output..

On a related note, how would you feel about implementing + and - to the --fields option? For example, running `fleetctl list-units --fields -tmachine,+hash` would hide the tmachine field but add the hash field. Useful?