dhcpv6-ia: split statefile handling to separate file

dhcpv6-ia: split statefile handling to separate file

dhcpv6-ia.c is already quite long, which makes it harder to work with.
In addition, the whole statefile machinery isn't specific to the dhcpv6
server, but shared by the dhcpv4 and dhcpv6 servers, so split it out to
its own file.

This is in preparation for some more hacking on the statefile handling.

The copyright headers on statefiles.[ch] are the result of some git
archeology, and I've taken the liberty of putting them in REUSE format,
using only the initial copyright year for each contributor (one of the
recommendations from the REUSE project).

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