Kit: netcfg: add explicit IP mask routes

Kit: netcfg: add explicit IP mask routes

macOS will use the wrong source address unless we add explicit routes
that mention the self-pointing gateway. Actually, it won't add any
implicit routes on its own, so in order to route the masks of the
addresses, we have to add our own routes explicitly.

However, this still doesn't fix the problem while inside of the network
extension, even though it works outside it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>