Kit: Adapter: use more reliable utun detection technique

Kit: Adapter: use more reliable utun detection technique

Rather than hoping that the AF_SYSTEM fd is of type utun, and then
calling "2" on it to get the name -- which could be defined as something
else for a different AF_SYSTEM socket type -- instead simply query the
AF_SYSTEM control socket ID with getpeername. This has one catch, which
is that the ID is dynamically allocated, so we resolve it using the
qualified name. Normally we'd make a new AF_SYSTEM socket for this, but
since that's not allowed in the sandbox, we reuse the AF_SYSTEM socket
that we're checking. At this point in the flow, we know that it's a
proper AF_SYSTEM one, based on the first sockaddr member; we just don't
know that it's a utun variety.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>