-harden get_single_ipv6
-support IPv4-compatible IPv6 address and IPv4-mapped IPv6 address parsing
remove unused ipv6expand
now can be trivially replaced by st_printf("%I0", ....);

Signed-off-by: etienne basset <etienne.basset@numericable.fr>
[BUG] fix IPv6 comparison function
IPv6 ARE filled as ushort[8], so must be compared in that order
using u32[4] can lead to endianness BUGS

Signed-off-by: etienne basset <etienne.basset@numericable.fr>