wg-quick: darwin: properly parse netstat -nr output

wg-quick: darwin: properly parse netstat -nr output

Looking at the source [1], it appears that the interface is always the
4th argument, regardless of the family:

C1.	if (fam != old_fam) // FALSE

P1.	p_sockaddr(params, &addr.u_sa, &mask.u_sa, rtm->rtm_flags,
	    WID_DST(addr.u_sa.sa_family));

P2.	p_sockaddr(params, rti_info[RTAX_GATEWAY], NULL, RTF_HOST,
	    WID_GW(addr.u_sa.sa_family));

C2.	if (params->lflag && (rtm->rtm_addrs & RTA_IFA)) // FALSE

P3.	p_flags(rtm->rtm_flags, "%-10.10s ");

C3.	if (params->lflag) // FALSE

P4. 	printf("%*.*s", WID_IF(addr.u_sa.sa_family),
		WID_IF(addr.u_sa.sa_family), ifname);

Because C1, C2, and C3 evaluate to false, interface is always in the 4th
argument.

[1] https://github.com/apple-oss-distributions/network_cmds/blob/97e27e6244c16d399bfeb254315ddc5828711c56/netstat.tproj/route.c#L328

Reported-by: Florian Uekermann <florian@uekermann.me>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>