This is the first implementation of the concept ot "TIME=" on the
configuration of ppsi.  Time-related code that used to live in arch-unix
is now split into its own directory. If you have special timing hardware,
you can write your own pp_network_operations and pp_time_operations
in your own directory and select them using TIME= .

We'll do Linux-specific hardware timestamping soon, and WhiteRabbit as
well.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>