tun_netbsd: support unpatched kernels for IFHEAD

This previously assumed a user's kernel was patched a la
http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=55463, making
it effectively useless for most people. Since we know IFHEAD will always
be one of 0 (false) or something that isn't 0 (true), we can generalize
things a bit more.

Suggestion provided by artemis
https://artemis.sh/2022/02/16/tailscale-on-netbsd-proof-of-concept.html

tun/tun_netbsd.go

@@ -164,7 +164,7 @@ func CreateTUN(name string, mtu int) (Device, error) {
         uintptr(unsafe.Pointer(&ifheadmode)),
     )
 
-    if errno != 0 || ifheadmode != 1 {
+    if errno != 0 || ifheadmode == 0 {
         tunfile.Close()
         return nil, fmt.Errorf("Unable to validate IFHEAD mode: %v (ifheadmode = %d)", errno, ifheadmode)
     }