debug credential audit trail

[flihp]

#142 raises questions about the constraints we want / need to enforce when signing debug credentials. The comparison is made w/ our CA operations in that they prevent us from issuing certs w/ identical serial numbers or the same subject fields. Similar (or dissimilar) constraints may be required of debug credentials. Some questions we should consider: Do we want our tools to prevent us from issuing a DC that's identical to a previously issued one? Other considerations?