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
Sounds a bit overly complex and trouble-prone to me - at that stage why not just explicitly name all the fields? and it ties us to the default output fields more closely.

I'd entertain a PR but wouldn't do it myself.