Read host keys from environment variables

[Philipp Marek]

>> One point I see here is that in case someone breaks into the 
>> container,
>> there's now a nicely defined API to fetch the host keys from,
> 
> No, the agent does not provide access to the raw keys.  It will sign
> challenges on your behalf, but it won't cough up the keys themselves.

Yeah, right, that's what I meant.


> You *could* probably pick the keys out of memory with a debugger
> (they're encrypted to mitigate rowhammer-style recovery attacks but a
> root-equivalent user could defeat that pretty easily).  That said, if
> you are in a position to do that to ssh-agent, you could just as
> easily do that to sshd itself.

Hmmm, host-keys in environment variables should do something similar 
then,
which makes the patch larger than +-10 lines ;/