pkgdev bugs: automatically search for and block open security bugs on --stablereq #220

I see this happen frequently enough that I think it is worth adding functionality for this to pkgdev. Some background, when security bugs are labeled as stable? (awaiting a stablereq), there are times when they were stabled days or sometimes even weeks before, resulting in lost time on the security team's end.

I'm willing to work on this myself.