Add compat.linux.ignore_ip_recverr sysctl. This is a workaround
for missing IP_RECVERR setsockopt(2) support. Without it, DNS
resolution is broken for glibc >= 2.30 (glibc BZ #24047).

From the user point of view this fixes "yum update" on recent
CentOS 8.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23234