dnstable: Add dnstable_entry_set_iszone() function

This function stores a boolean "iszone", which we'll make use of in the
formatting functions.
entry: dnstable_entry_to_text(): Alter output based on iszone flag

If 'iszone' is set on the entry object, we need to generate a slightly
different text format.
entry: dnstable_entry_to_json(): Alter output based on iszone flag

If 'iszone' is set on the entry object, we need to use the keys
'zone_time_first' and 'zone_time_last' instead of 'time_first' and
'time_last'.
Merge pull request #2 from farsightsec/branches/zone-time-fixes

Branches/zone time fixes