networking: enable "multi on" in resolver settings

[Mic92]

Motivation for this change

this allows to return ipv4/ipv6 addresses for the same host in /etc/hosts.
fixes #19148

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@Mic92, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ardumont, @edolstra and @wkennington to be potential reviewers.

[nhooyr]

We're also missing the manpage for host.conf(5) so we should add that.

[Mic92]

I have this manpage on nixos unstable. I have manpages in system-packages.

[vcunat]

@nhooyr: how so? Our standard man-pages package does contain it.

[bennofs]

@nhooyr man host.conf works on my NixOS system:

$ man -w host.conf 
/nix/store/29x1w0qmbxkns17zzj2f6ldnc3xp2dl3-man-pages-4.07/share/man/man5/host.conf.5.gz

[nhooyr]

Ah sorry, I didn't have the man-pages package.