fmt_ip6if formats an IPv6 number in ASCII representation from ip and
writes the result into dest. It returns the number of bytes
fmt_ip6if will apply "::" compression to the output.
If ip is an IPv4-mapped IPv6 address, fmt_ip6if will output the last
4 bytes as IPv4 number in dotted-decimal notation.
If dest equals FMT_LEN (i.e. is zero), fmt_ip6if returns the number
of bytes it would have written.
fmt_ip6 does not append \0.
ip6.h defines the integer IP6_FMT to be big enough to contain every
possible fmt_ip6 output plus \0. However, for fmt_ip6if you need to
add 1 for the % and IF_NAMESIZE from <net/if.h> for the interface