statefiles: unify host6 writing

statefiles: unify host6 writing

The statefile is a mix of state comments and hostsfile entries, reuse the
hosts writing logic when writing the statefile to reduce duplication.
(note that this has a tiny extra cost in that inet_ntop() is called
twice, that'll go away later when we drop the hostsfile style lines from
the statefile).

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/302
Signed-off-by: Álvaro Fernández Rojas <[email protected]>