dhcpv6-ia: fix invalid IPv6/hostname entries in statefile

Statefull IPv6 addresses are assigned to DHCPv6 clients from a
local prefix based on criteria which need to be fulfilled.
One of those criteria requires the IPv6 address still having
preferred lifetime which was not checked when constructing
the contents of the statefile.
Another criteria requires the prefix length not being smaller
than or equal to 64 when ra_management equals 0 or 1 which was
neither correctly checked when constructing the statefile.
This can lead to invalid hostname/IPv6 entries in the statefile
and thus possible connectivity issues when using DNS resolving.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>