When sending client requests to a close and fast server, it is possible
that a response will be received before the HW transmit timestamp of
the request itself. To avoid processing of the response without the HW
timestamp, monitor events returned by select() and suspend reading of
packets from the receive queue for up to 200 microseconds. As the
requests are normally separated by at least 200 milliseconds, it is
sufficient to monitor and suspend one socket at a time.
Enable SCM_TIMESTAMPING control messages and the socket's error queue in
order to receive our transmitted packets with a more accurate transmit
timestamp. Add a new file for Linux-specific NTP I/O and implement
processing of these messages there.
