fmt_ip6c formats an IPv6 number in ASCII representation from ip and
writes the result into dest. It returns the number of bytes
fmt_ip6c will apply "::" compression to the output.
If ip is an IPv4-mapped IPv6 address, fmt_ip6c will output it as
If dest equals FMT_LEN (i.e. is zero), fmt_ip6c returns the number
of bytes it would have written.
fmt_ip6c does not append \0.
For convenience, ip6.h defines the integer IP6_FMT to be big enough to
contain every possible fmt_ip6c output plus \0.