ntp: add pre-NTPv5 experimental extension field

ntp: add pre-NTPv5 experimental extension field

Add an experimental extension field for some features that were proposed
for NTPv5. Higher-resolution root delay and dispersion (using 28-bit
fraction) are added. A monotonic receive timestamp will allow a
frequency transfer between the server and client. The client will be
able to separate the server's time corrections from frequency
corrections by tracking the offset between the real-time and monotonic
receive timestamps.

The field has a type of 0xF323 from the new experimental range proposed
by the NTP working group. Include a magic 32-bit value in the field to
avoid interoperability issues if a different implementation choses the
same type for its own experimental field. The value will be changed on
incompatible changes to avoid issues between two different chrony
versions.