statefiles: simplify dhcpv6_ia_write_hostsfile()

statefiles: simplify dhcpv6_ia_write_hostsfile()

Use a dirfd to keep track of the hostsfile directory, also make sure
that the directory is created if necessary (this is in preparation for
writing per-interface hostsfiles).

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