Proposal for hardening agent forwarding

[Mitchell Blank Jr]

Hello.

This week I've been experimenting with some hardening of the agent-forwarding
process.  I know there have been other proposals in the past, but I thought
I'd share what I have in case they are of any upstream interest.

For easier review (and to spare your inboxes) I just opened it as a PR
on the openssh-portable github mirror here: https://github.com/openssh/openssh-portable/pull/233

In short it's similar functionality to Timo Weingärtner's ssh-agent-filter
tool that many are probably already familiar with, but integrated directly
into the openssh client.

I just did this for my own use-case, but if some of it is interesting as
an upstream addition feel free to re-use whatever parts you want.

-Mitch