ntp: add support for NTP over PTP

ntp: add support for NTP over PTP

Allow NTP messages to be exchanged as a payload of PTP messages to
enable full hardware timestamping on NICs that can timestamp PTP packets
only. Implemented is the protocol described in this draft (version 00):

https://datatracker.ietf.org/doc/draft-mlichvar-ntp-over-ptp/

This is an experimental feature. It can be changed or removed in future.
The used PTP domain is 123 and the NTP TLV type is 0x2023 from the "do
not propagate" experimental range.

The ptpport directive enables NTP-over-PTP as a server and as a client
for all sources that have the port option set to the PTP port. The port
should be the PTP event port (319) to trigger timestamping in the
hardware.

The implementation is contained to ntp_io. It is transparent to
ntp_core.