Add DragonFly BSD support and improve FreeBSD support

• Support DragonFly BSD for TUN creation and destroy in the same way as FreeBSD.
• No need to load the if_tun kernel module, because the ifconfig(8) utility on FreeBSD and DragonFly BSD will auto load it if necessary. This additionally improves FreeBSD support for FreeBSD >=13 because the if_tap and if_tun modules have been merged into if_tuntap.
• Fix TUN address deletion syntax.
• Use [[:space:]] instead of \s to support BSD's awk(1).
• Improve the determination of default route.

Signed-off-by: Aaron LI aly@aaronly.me