BSD + macOS: fix send() on loopback and utun devices

[micolous]

This pull request fixes the send() function on loX (if_lo) and Darwin utunX (if_utun), by including the Loopback header unless on Darwin/macOS with a device that looks like tuntaposx's TUN device.

My previous change (#2584) of mine operated under the assumption that a DLT_NULL (Loopback) device doesn't need any headers on BSD, because tuntaposx on Darwin didn't work with it. At the time, I hadn't tested this on BSD, but the true story seems to be a bit more complicated:

• Prior to FreeBSD 10.0, bpf couldn't send packets to if_lo interfaces due to a kernel bug.
• FreeBSD 13.0's if_lo needs the Loopback header.
• FreeBSD 13.0's if_tuntap is broken due to a kernel bug, but should need the Loopback header once that's fixed.
• Darwin's if_lo and if_utun need the Loopback header.
• tuntaposx's tun interface needs no headers (per Migrate tuntap into own module, add ICMPEcho_am and bugfixing #2584), because they seem to interpret NULL as "no headers".

Without this change:

• running Wireshark against a utun has missing headers, and the first 4 bytes of the packet are swapped between network and host byte order, and reports an "Unknown" packet type of 0x001c.
• an application on the far side of the utun device gets bad packets (because it's missing the DLT_NULL header)

Long term it looks like Apple is pushing userspace applications towards utun and away from third-party kernel extensions like tuntaposx. I'm working on another PR which adds utun support for TunTapInterface, but that's not quite ready yet (#3258).

Tests: I've added a test to bpf.uts which sends an ICMP ping packet to localhost on IPv4, and tries to sniff for that packet being sent and a response from the kernel. This seems to work on FreeBSD 13.0-RELEASE and macOS 11.4.

[codecov]

Codecov Report

Merging #3257 (f9656e3) into master (a6bf8dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3257   +/-   ##
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  Coverage   52.38%   52.38%           
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  Files           9        9           
  Lines        1365     1365           
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  Hits          715      715           
  Misses        650      650           

[micolous]

I'm still in the process of updating the comment per feedback on #3258.

However, I've found that FreeBSD has buggy handling of DLT_NULL for tun interfaces, and I'm still trying to get my head around all that.

[micolous]

After the tests finish, this is ready again.

[gpotter2]

Pretty cool PR, thanks !