2001:DB8::8:800:200C:417A
Fix #8
::13.1.68.3
Fix #9
Treat `2001:DB8:0:0:8:800:200C:417A` as valid.
This change introduces `isinetaddr6`, a function that validates IPv6 addresses. The interface is identical to isinetaddr, where 0 is returned for an invalid address and 1 is returned for a valid address.