nixos: add BitTorrent Sync service module

[thoughtpolice]

This adds a BitTorrent Sync service module with all the supported configuration options from the JSON configuration file. You can use it through a UI or through the config file to sync specific directories (the two are mutually exclusive).

This module also has one interesting aspect I haven't seen exploited in a NixOS module before: multi-instance systemd units. Note in particular the btsync@ unit.

This allows the administrator to selectively start instances of the btsync service on-demand for a user with their own configuration file. For example, I can have my own config file at /home/a/.config/btsync.conf, and the administrator can start my instance with

$ systemctl start btsync@a

which will start an independent service under my user with my configuration file. For the record, I took this idea from ArchLinux.

I suppose in an ideal world, instead of multi-instance services we would use systemd user services (ref #1689) but as that's not done yet, I find this works adequately. I'm open to other recommendations, however.

[edolstra]

This should note that secrets are not very secret in the Nix store.

[shlevy]

@thoughtpolice Are you still working on this? Can you address the comments?

[thoughtpolice]

@shlevy Done. This is rebased on top of master with the latest UID changes, and is Ready To Merge.

[thoughtpolice]

This change also addresses all previous comments by @edolstra and @iElectric

[domenkozar]

Looks good to me

[shlevy]

FWIW we already had for example

nixpkgs/nixos/modules/services/networking/websockify.nix

Lines 37 to 45 in 5c1f8cb

	systemd.services."websockify@" = {
	description = "Service to forward websocket connections to TCP connections (from port:to port %I)";
	script = ''
	IFS=':' read -a array <<< "$1"
	${pkgs.pythonPackages.websockify}/bin/websockify --ssl-only \
	--cert=${cfg.sslCert} --key=${cfg.sslKey} 0.0.0.0:''${array[0]} 0.0.0.0:''${array[1]}
	'';
	scriptArgs = "%i";
	};

;) But looks good, merging.