Adds support for hardware timestamping of received Ethernet packets.
The timestamp is available to applications using socket option SO_TIMESTAMP.
Optionally, the Ethernet PTP timer can be used as system high-resolution RTC.
In this mode it supports fine resolution rate adjustment.
Alternatively other time source for CLOCK_REALTIME can be used, and the
PTP timestamps are converted by sampling the clocks and computing the
difference. This results in a few microseconds of uncertainty.
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