ssh-agent: autoload identities not already loaded (#7174)

With this PR the ssh-agent plugin checks the `ssh-add -l` output for the
identities added, and adds all those specified by the user that haven't been
added yet.

We also decouple the logic of starting ssh-agent from the logic of adding
identities, meaning that even if ssh-agent has been started by some other means
(like launchd) we can still ssh-add the user's identities.

Fixes #3019
Fixes #6979