Add support for InetAddressIPv6

[kbabioch]

InetAddressIPv6 as defined in RFC4001 (e.g. SNMP) deals with IPv6 addresses in the following way:

fe:80:00:00:00:00:00:00:c2:ee:fb:ff:12:ed:12:96

The details can be found here:

InetAddressIPv6 ::= TEXTUAL-CONVENTION
    DISPLAY-HINT "2x:2x:2x:2x:2x:2x:2x:2x"
    STATUS       current
    DESCRIPTION
        "Represents an IPv6 network address:

           Octets   Contents         Encoding
            1-16    IPv6 address     network-byte order

         The corresponding InetAddressType value is ipv6(2).

         This textual convention SHOULD NOT be used directly in object
         definitions, as it restricts addresses to a specific format.
         However, if it is used, it MAY be used either on its own or in
         conjunction with InetAddressType, as a pair."
    SYNTAX       OCTET STRING (SIZE (16))

Basically there are 16 octets encoded as hex divided by :. Support for this should be added, as this is defined in an RFC. It defines an IPv6 address in an unique way, so there should be no ambiguities, etc. and the implementation should be pretty much straight forward.