dns: add options to control automatic addition of edns0 and trust-ad
Summary
Introduce new options _skip-edns0
and _skip-trust-ad
to prevent
the automatic addition of edns0
and trust-ad
when using caching plugins. The initial
underscore indicates that the option is internal and is not written
into resolv.conf.
Purpose
Options edns0
and trust-ad
are automatically added when using
caching plugins such as dnsmasq and systemd-resolved. In some cases,
those options can break resolution due to non-conforming resolvers,
and there is no way to disable them. This MR provides a solution to that problem, reported in #1393 (closed)
I couldn't find any better names for the options, suggestions are welcome.