Add Rule.Type to allow adding/listing unreachable (RTN_UNREACHABLE) rules?

[bradfitz]

The Rule type has no Type field or similar to permit creating RTN_UNREACHABLE rules.

Looking at func ruleHandle, there doesn't seem to be a way to set the msg.Type to anything other than unix.RTN_UNSPEC, unix.RTN_UNICAST, or nl.FR_ACT_GOTO.