Attempt to support NetBSD

This code actually consolidates a lot of the BSD code together, and even setting the interface MTU with SIOCSIFMTU seems to work fine.

What doesn't work though is setting the interface address using SIOCSIFADDR_IN6, which I attempted to plagiarise from the Darwin code.

As a fallback, ifconfig is used, which solves the problem enough to get it working.

src/yggdrasil/tun_netbsd.go

@@ -0,0 +1,10 @@
+package yggdrasil
+
+func getDefaults() tunDefaultParameters {
+	return tunDefaultParameters{
+		maximumIfMTU:     9000,
+		defaultIfMTU:     9000,
+		defaultIfName:    "/dev/tap0",
+		defaultIfTAPMode: true,
+	}
+}