Notifications are only sent for the top object, and not for individual
links. This should be enough for the most obvious cases where somebody
just cares about the effective set of servers.

Fixes #13721.

While at it, constify the argument.

This matches what is done in networkd very closely. In fact even the
policy descriptions are all identical (with s/network/resolve), except
for the last one:
resolved has org.freedesktop.resolve1.revert while
networkd has org.freedesktop.network1.revert-ntp and
org.freedesktop.network1.revert-dns so the description is a bit different.

This doesn't matter much, but let's just do the loop once and allocate
the populate the result set on the fly. If we find an error, it'll get
cleaned up automatically.

This also changes the type from char[IF_NAMESIZE] to char*.
By changing the type, now resolved-link.h can drop the dependency to
the header net/if.h.

These lines are generally out-of-date, incomplete and unnecessary. With
SPDX and git repository much more accurate and fine grained information
about licensing and authorship is available, hence let's drop the
per-file copyright notice. Of course, removing copyright lines of others
is problematic, hence this commit only removes my own lines and leaves
all others untouched. It might be nicer if sooner or later those could
go away too, making git the only and accurate source of authorship
information.

This part of the copyright blurb stems from the GPL use recommendations:

https://www.gnu.org/licenses/gpl-howto.en.html

The concept appears to originate in times where version control was per
file, instead of per tree, and was a way to glue the files together.
Ultimately, we nowadays don't live in that world anymore, and this
information is entirely useless anyway, as people are very welcome to
copy these files into any projects they like, and they shouldn't have to
change bits that are part of our copyright header for that.

hence, let's just get rid of this old cruft, and shorten our codebase a
bit.

PrivateDNS is not considered a good name for this option, so rename it to DNSOverTLS

Like with DNSSec, make PrivateDNS configurable per link, so you can have trusted and untrusted links.