Support NTP 2036 year base

[gavv]

Currently we don't implement this bit from RFC:

      As the NTP timestamp format has been in use for the last 17 years,
      it remains a possibility that it will be in use 40 years from now
      when the seconds field overflows. As it is probably inappropriate
      to archive NTP timestamps before bit 0 was set in 1968, a
      convenient way to extend the useful life of NTP timestamps is the
      following convention: If bit 0 is set, the UTC time is in the
      range 1968-2036 and UTC time is reckoned from 0h 0m 0s UTC on 1
      January 1900. If bit 0 is not set, the time is in the range 2036-
      2104 and UTC time is reckoned from 6h 28m 16s UTC on 7 February
      2036. Note that when calculating the correspondence, 2000 is not a
      leap year. Note also that leap seconds are not counted in the
      reckoning.

https://datatracker.ietf.org/doc/html/rfc2030

Affected functions:

• ntp_2_unix()
• unix_2_ntp()
• ntp_equal_delta()