v2 human readable signature status

[David Bremner]

Based on the feedback from dkg, this is a less intrusive change to the
API, and just expands out the error flags.

This obsoletes [1]. I think that the current status values can be
recovered from the gmime 3.0 signature status, but it remains a bit of
an open question whether we should mimic the behaviour of gpgme in
reporting multiple good states.  See [2] for a discussion of that.
Given we can preserve our current api, perhaps we can just defer that
change to a later date.

[1] id:20170531114535.10506-1-david at tethera.net
[2] id:87k24x6qvq.fsf at tesseract.cs.unb.ca