1. in ticket_lifetime() calculate the remaining lifetime
of the ticket rather than the requested lifetime.
2. in renew_func(), attempt to renew if the tickets are
renewable rather than only if --renewable is specified.
3. fix the call to renew_validate() in renew_func() to
specify renewable tickets if the original tickets are
renewable rather than only if --renewable is specified.
4. stop printing constant warnings to the terminal about
how tickets cannot be obtained if they expire, cannot
be renewed and we can't non-interactively obtain fresh
ones. We limit it to a single warning.
5. after the tickets expire, we backoff the requests to
obtain fresh tickets exponentially.