Pubkey auth: keys from ssh-agent are used even if HostConfigEntry.isIdentitiesOnly() is true

UserAuthPublicKeyIterator unconditionally includes keys from the SSH agent if there is an SshAgentFactory. This should be done only if !HostConfigEntry.isIdentitiesOnly().

Also, there is a completely superfluous requirement that the SshAgentFactory return a non-null SshAgent in that iterator. UserAuthPublicKeyIterator.initializeAgentIdentities() could just return null in that case.

Furthermore it would be useful if the session was passed through to SshAgentFactory.createAgent().

Finally, the ordering of keys from different sources seems to be strange. Agent keys always come first, then the session keys. The session keys are the ones set explicitly, plus then the default keys. So the order is <agent, explicit, default>. I think this should be <explicit, agent, default>.