Add DragonFly BSD support and improve FreeBSD support

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
(the `-n` option suppresses this behavior; see man page for details).
This feature was added to `ifconfig(8)` in 1999:
https://github.com/freebsd/freebsd/commit/4d16916f800d4c5ee37fb3c93ca6981ad9b33eb3

In addition, FreeBSD 13 has merged `if_tun` and `if_tap` into a single
module named `if_tuntap`.  So this change improves the FreeBSD
support.

Signed-off-by: Aaron LI <aly@aaronly.me>
Signed-off-by: Daniel Lenski <dlenski@gmail.com>