ntp: return status from NCR_ProcessKnown()
ntp: update number of sources when removing all sources

Also, rehash the records after removal and split cleaning of the source
record to a separate function.
ntp: limit number of pool sources

A new option can be now used in the pool directive: maxsources sets the
maximum number of sources that can be used from the pool, the default
value is 4.

On start, when the pool name is resolved, chronyd will add up to 16
sources, one for each resolved address. When the number of sources from
which at least one valid reply was received reaches maxsources, the
other sources will be removed.