Fix sentinel position in sample u2u server

Print "Server started" after calling listen(), or there wil be a race
where the client tries to connect before there is a listen queue and
gets ECONNREFUSED.
Fix timing edge cases in t_renew.py

When we are testing maximum renewable lifetimes, the KDC might process
the request at a later time than the request time (typically by no
more than one second).  So we need to ask for a ticket lifetime longer
than the maximum renewable lifetime, not equal to it, or we risk
getting a just-barely-renewable ticket instead of a non-renewable one.
Also fix a couple of typos in comments.