wireguard-tools: ipc: freebsd: use AF_LOCAL for the control socket

AF_INET assumes that IPv4 is built into the kernel, but it's completely
valid to build a kernel without it. unix(4) sockets, on the other hand,
are not-optional in the kernel build. Given that interface ioctls can
be invoked on any kind of socket, switch to the safer one to avoid
breaking on IPv6-only kernels.

Reported and tested by: ivy
MFC after: 3 days