Reintroduce NTP mode7 for IEEE 1588 PTPd interop
It should be possible to selectively re-enable Mode 7 packets. This allows PTPd to take time from NTPd on the node. This is necessary as PTPd does not have any reference clock drivers (e.g. GPS) of its own.
I use this to distribute time from a pfSense node with a GPS card through IEEE 1588. Security is fine, provided mode7 can be restricted to loopback with an ntpd ACL.
(This requirement should eventually go away once PTPd no longer requires mode7 NTP control.)