add utun for darwin

[majestrate]

add support for apple's non standard utun network interace creation mechanism.

[tleguern]

Call for testing: @chipot or @ntnmrndn 😃

[tleguern]

Variable declarations should respect ANSI C and thus be at the top of the current block.

[ntnmrndn]

Hi, sorry for the delay, I'm starting to review the PR.

It doesn't compile on my computer:

libtuntap/tuntap-unix-darwin.c:63:39: error: use of undeclared identifier 'errno'

Adding the include #include <sys/errno.h> solves the issue
As for the tests, we went from

14 - tuntap.test14 (Failed)
16 - tuntap.test18 (Failed)

to

2 - tuntap.test02 (Failed)
6 - tuntap.test06 (Failed)
8 - tuntap.test08 (Failed)
10 - tuntap.test10 (Failed)
12 - tuntap.test12 (Failed)
14 - tuntap.test14 (Failed)
16 - tuntap.test18 (Failed)
18 - tuntap.test22 (Failed)
20 - tuntap.test24 (Failed)
22 - tuntap.test26 (Failed)

[tleguern]

@ntnmrndn: It is clear every tun operations are failing. Can you run the failing regress tests individually and share the output please ?

[ntnmrndn]

"Can't get interface values"

[tleguern]

I think the code needs to be split a bit further than this as the traditional ioctl calls are not going to work on utun interfaces. The code is currently able to create an utun device of unspecified number but can't manipulate it yet.