Complete rewrite of pam_ssh(8). The previous version was becoming hard
to maintain, and had security issues which would have required a major
rewrite to address anyway.

This implementation currently starts a separate agent for each session
instead of connecting each new session to the agent started by the first
one. While this would be a Good Thing (and the old pam_ssh(8) tried to
do it), it's hard to get right. I'll revisit this issue when I've had a
chance to test some modifications to ssh-agent(1).