Make register dump output more concise.

Add `show` argument that lets user decide how to deal with fields whose
value is 0:
* Show them all.
* Show them if they have a symbolic name.
* Don't show them.

Use underscores instead of whitespace in field names, so we don't need a
comma in the separator.

New format:
```
{datacount=2 cmderr=not_supported}
{version=0.13 anyresumeack=1 allresumeack=1 impebreak=1 authenticated=true anyhalted=1 allhalted=1}
```