Fix osx 1085 ipv6 dns in resolv

I just tried to connect to a host with Sshuttle and it failed. The error is below (generated with -v). The pull request fixes this problem (seen on OS X 10.8.5).

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>> ipfw -q add 12300 divert 12300 udp from any to 2001:4888:68:ff00:608:d::/32 53 not ipttl 42
ipfw: bad netmask ``4888:68:ff00:608:d::/32''
firewall manager: undoing changes.
>> ipfw -q delete 12300
fatal: ['ipfw', '-q', 'add', '12300', 'divert', '12300', 'udp', 'from', 'any', 'to', '2001:4888:68:ff00:608:d::/32', '53', 'not', 'ipttl', '42'] returned 65