Add support for jump hosts

The jump-session function is used to obtain a session that can be
connected across jump hosts.

The `the-session` function is added to obtain a jsch session from a
connected jump-session, or a connected jsch Session, and can be used in
code that wants to support jump sessions.
Update tools.logging to 0.2.6
Enable copying identities between agents

The copy-identities can be used to copy identities from one agent to
another.  This is useful to allow copying identities from a system agent
to a non system agent.
Enable introspection of sessions

Adds the session? predicate for testing for a Session object, an the
session-hostname and session-port functions for querying a session.