ntp: T5692: add support to configure leap second behavior

• set service ntp leap-second [ignore|smear|system|timezone]

Where timezone is the new and old default resulting in adding "leapsectz right/UTC"
to chrony.conf. The most prominent new option is "smear" which will add

leapsecmode slew
maxslewrate 1000
smoothtime 400 0.001 leaponly

to chrony.

See https://chrony-project.org/doc/4.3/chrony.conf.html leapsecmode for
additional information