This commit adds one major new option to the znc module:
services.znc.config. It can represents the full ZNC configuration as a
Nix value, see its description for more info. It acts as a more
flexible alternative for the older services.znc.confOptions.* options.
Effectively this also allows the user to change previously fixed options. Along
with this comes a nicely generated config file with proper indentation.

Unless I made a mistake, these changes are 100% backwards compatible.

Other changes:

• Proper escaping in start script -> allows for special directory/file
  names
• All znc modules packaged in nixpkgs are added to the
  services.znc.modulePackages option by default, because they are very
  small (<100KB each)
• Clean up the module in general and split the old options into a
  separate file.
• Correct network.service -> network.target

It's now possible to:

• Have multiple users
• Set channels as detached
• Define QuitMsg
• And everything else that wasn't previously possible

The new configuration style is a direct translation of the xml-like znc config file syntax, all the options are described in the official ZNC wiki. It looks like this:

{
  services.znc = {
    enable = true;
    openFirewall = true;
    mutable = false;
    config = {
      LoadModule = [ "playback" ];
      Listener.l = {
        Port = 5000;
        IPv4 = true;
        IPv6 = true;
        SSL = true;
      };
      User.infinisil = {
        Admin = true;
        LoadModule = [ "push" ];
        QuitMsg = "Configuring ZNC, sorry for the joins/quits!";
        Nick = "infinisil";
        AltNick = "infinisi1";
        Ident = "infinisil";

        Network.freenode = {
          Server = "chat.freenode.net +6697";
          Chan = {
            "#nixos" = { Detached = true; };
            "#zfsonlinux" = { Disabled = true; };
          };
        };
      };
    };
  };
}

If you don't define the new config option, the legacy confOptions option will be used for it.

My better-znc-module branch contains the unsquashed version of this PR. It would probably have been nicer to have multiple commits because of the many changes.

Pinging the people who have toched the module in the past: @layus @mbrgm @vaibhavsagar @nocoolnametom @schneefux @lethalman

And pinging the maintainers of the package: @viric @schneefux @LnL7

Todo

• Test it some more
• Minimize the znc config in the example, might not need everything there
• I think in the old options there is one that's not used at all, remove it or deprecate it in case anybody set it in the hopes of it doing something?
• Maybe add a section in the docs regarding this. The services.znc.config description explains it pretty well already though.
• Fine tune option descriptions
• What to do with the bitlebee thing? -> Ain't gonna do anything!