Fix an issue where access.log would have the wrong timestamp.

The refactoring to use monotonic timestamps did not take into account
that the timestamps are used for the access log, so it'd contain
(roughly) time since boot instead of the actual time of day. Fixed by
measuring the offset between the two (although if the clock is wrong
at the time of logging, the connection time will be wrong -- as opposed
to if the clock is wrong at the time of connect, hard to say which is
the better). Reported by Joachim Tingvold.

While we are at it, change some of the timespec functions so that we
get slightly more precise timing in the logs (it could be about a second
off). For cosmetics only.