systemd: Backport systemd-resolved: use hostname for certificate validation in DoT

systemd: Backport systemd-resolved: use hostname for certificate validation in DoT

Widely accepted certificates for IP addresses are expensive and only
affordable for larger organizations. Therefore if the user provides
the hostname in the DNS= option, we should use it instead of the IP
address.

This fixes https://nvd.nist.gov/vuln/detail/CVE-2018-21029 per
suggestion https://github.com/systemd/systemd-stable/issues/72 .

CVE: CVE-2018-21029
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Steve Sakoman <steve@sakoman.com>