Kit: Adapter: iterate through all FDs to find UTUN

Kit: Adapter: iterate through all FDs to find UTUN

This is a bit of a kludge, until I find something better. We simply
iterate through all FDs, and call getsockopt on each one until we find
the utun FD. This works, and completes rather quickly (fd is usually 6
or 7). Rather than maintain the old path for older kernels, just use
this for all versions, to get more coverage. Other techniques involve
undocumented APIs; this one has the advantage of using nothing
undocumented.

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