lib/timeutil: add utilities to manage local/reference clock alignment

Provide data structures to capture a timestamp in two different
clocks, monitor the drift between those clocks, and using a base
instant with estimated drift convert between the clocks.

This provides the core technology to convert between system uptime and
an external continuous time scale like TAI (UTC without applying leap
seconds).

Signed-off-by: Peter A. Bigot <pab@pabigot.com>