Better trust chain output

[mcpherrinm]

Today, we dump all built trust chains.

That could be improved in a few ways:

• [ ] Don't print all of them out unless in --verbose
• [ ] Warn about un-needed intermediates (but be careful; they may be needed with other trust stores)
• [ ] Warn about un-needed roots served in trust chains
• [ ] some support for pinning leafs / intermediates

[matthyx]

I would even fail the validation if an intermediate is missing from the presented chain. Today it validates if the intermediate is present in the CA bundle (which is wrong).