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.

doc/chrony.conf.adoc

@@ -2365,7 +2365,7 @@ timestamping, which can be verified with the *ethtool -T* command. The list of
 capabilities should include _SOF_TIMESTAMPING_RAW_HARDWARE_,
 _SOF_TIMESTAMPING_TX_HARDWARE_, and _SOF_TIMESTAMPING_RX_HARDWARE_. Receive
 filter _HWTSTAMP_FILTER_ALL_, or _HWTSTAMP_FILTER_NTP_ALL_, is necessary for
-timestamping of received packets. Timestamping of packets received from bridged
+timestamping of received NTP packets. Timestamping of packets received on bridged
 and bonded interfaces is supported on Linux 4.13 and newer. When *chronyd* is
 running, no other process (e.g. a PTP daemon) should be working with the NIC
 clock.
@@ -2421,12 +2421,14 @@ Enables timestamping of received PTP packets.
 _none_::::
 Disables timestamping of received packets.
 {blank}:::
-The most specific filter for timestamping NTP packets which is supported by the
-NIC is selected by default. Some NICs can timestamp only PTP packets, which
-limits the selection to the _none_ filter. Forcing timestamping of all packets
-with the _all_ filter when the NIC supports both _all_ and _ntp_ filters can be
-useful when packets are received from or on a non-standard UDP port (e.g.
-specified by the *port* directive).
+The most specific filter for timestamping of NTP packets supported by the NIC
+is selected by default. Some NICs can timestamp PTP packets only. By default,
+they will be configured with the _none_ filter and expected to provide hardware
+timestamps for transmitted packets only. Timestamping of PTP packets is useful
+with NTP-over-PTP enabled by the <<chrony.conf.adoc#ptpport,*ptpport*>>
+directive. Forcing timestamping of all packets with the _all_ filter could be
+useful if the NIC supported both the _all_ and _ntp_ filters, and it should
+timestamp both NTP and PTP packets, or NTP packets on a different UDP port.
 {blank}::
 +
 Examples of the directive are:
@@ -2519,6 +2521,32 @@ e.g.:
 pidfile /run/chronyd.pid
 ----
 
+[[ptpport]]*ptpport* _port_::
+The *ptpport* directive enables *chronyd* to send and receive NTP messages
+contained in PTP event messages (NTP-over-PTP) to enable hardware timestamping
+on NICs which cannot timestamp NTP packets, but can timestamp unicast PTP
+packets. The port recognized by the NICs is 319 (PTP event port). The default
+value is 0 (disabled).
++
+The NTP-over-PTP support is experimental. The protocol and configuration can
+change in future. It should be used only in local networks and expected to work
+only between servers and clients running the same version of *chronyd*.
++
+The PTP port will be open even if *chronyd* is not configured to operate as a
+server or client. The directive does not change the default protocol of
+specified NTP sources. Each NTP source that should use NTP-over-PTP needs to
+be specified with the *port* option set to the PTP port. To actually enable
+hardware timestamping on NICs which can timestamp PTP packets only, the
+*rxfilter* option of the *hwtimestamp* directive needs to be set to _ptp_.
++
+An example of client configuration is:
++
+----
+server foo.example.net port 319
+hwtimestamp * rxfilter ptp
+ptpport 319
+----
+
 [[sched_priority]]*sched_priority* _priority_::
 On Linux, FreeBSD, NetBSD, and Solaris, the *sched_priority* directive will
 select the SCHED_FIFO real-time scheduler at the specified priority (which must