Make block local work in IPv6 as well

[roop]

Fixes #323.

The actual code changes are in eduvpn/tunnelkit (Commit).

The original code used to do:

• Find the default route
• Find the broadest route routing into the gateway of the default route
• Partition that route into two routes (to make that override the original route), routing into the tunnel

In IPv6, the following things make this not work:

• Some default routes route into non-existant "utun" interfaces (maybe not cleaned up by the OS yet)
  • Solution: Need filter out these routes
• The broadest route can be a link-local address
  • Solution: Need to find a link-layer address that routes into the interface of the default route (broadest, if there are many)

To test it:

• Connect to a wifi network that allocates a public IPv6 address to our machine (say A)
• In another machine (say B), connect to the same wifi network
• run ifconfig en0 in Machine B, note down the secured IPv6 address
• From Machine A, ping6 to the Machine B's IPv6 address (should work)
• In Machine A, connect to a VPN server that sets block-local
• From Machine A, ping6 to the Machine B's IPv6 address (should not work while tunnel is on)