ntp: update NTP-over-PTP support

ntp: update NTP-over-PTP support

Following the latest version of the draft, accept NTP messages in both
PTPv2 and PTPv2.1 messages, accept sync messages in addition to delay
request messages, and check the minorSdoId field in PTPv2.1 messages.

Transmitted messages are still PTPv2 delay requests.

Don't switch to the organization-specific TLV yet. Wait for the NTP TLV
subtype and Network Correction extension field to be assigned by IANA to
avoid an additional break in compatibility.