nixos: add 'networking.dnsExtensionMechanism' option to enable edns0 (for DNSSEC)

Set this option to 'true' (default: 'false') to enable extension mechanisms for
DNS (EDNS) in your local glibc resolver. This is required for supporting
DNSSEC, for example.

Implementation detail: the patch changes assignments to "resolv_conf_options"
to use "+=" instead of "=" to ensure that multiple users of that variable don't
overwrite each other. The generated config file is a shell script, after all,
so this should work fine.

Closes https://github.com/NixOS/nixpkgs/issues/12470.