Introduce session params

[pprindeville]

Note: This fix needs to be rebased after PR #72 is merged.

If you have a process which involves multiple simultaneous connections to different Tacacs+ servers (with different parameters, such as the secret, the session-id, the sequence #, etc) then having these values stored in globals is a severe hindrance.

This changeset presents a simplification of the API, containing all per-session data in a per-session object.

It also means that as new parameters get added in the future, this can be done with less source-level churn (though a version bump and recompile will still be required).

Lastly, there's the possibility of allocating extra data at the end of the session object for user-defined information to be stored; this could be as simple as a pointer to another object, or an entire nested object:

    sess = tac_session_alloc_extra(sizeof(*cookie));
    cookie = tac_session_get_user_data(sess);
    cookie->server_no = n;
    cookie->callback = got_response;

The callback data will be more useful in a subsequent patch which introduces event-driven querying using libevent but was introduced here since the tac_session is being introduced here.

[pprindeville]

Still have more PR's to submit, but they are all predicated on the outstanding (unmerged) PR's.

[pprindeville]

Only the last 2 commits apply: the rest are from branching from redux-packet-marshalling and will go away with a rebase once that gets merged to master.

[pprindeville]

Rebased to PR #72 after it was rebased to master today.

[pprindeville]

Merge PR #80, #72, and #82 (in that order) before this one.