bird-lg: init at 2020-05-20-unstable

modules/wireguard-systemd: init

Rather than using an ad-hoc configuration on my new machines, upstream
this here.  Eventually I might push this to Nixpkgs if I work out the
tests and integration with the scripted variant, or if someone else
volunteers.

matrix-appservice-irc: Add NixOS module

Inspired by, but not a copy of, github:NixOS/nixpkgs#95854.

trust-dns: init at 0.19.5

modules/ipfs-cluster: init

modules/dma: init

Split packages and modules off nixos-configurations

This will allow me to make them public while keeping the NixOS
configuration private.

Some reorganization was made to 1) be compatible with NUR, even though
I don't currently use it; and 2) mirror the nixpkgs taxonomy, to more
easily move packages upstream later on.

All package and NixOS module expressions were taken from
nixos-configurations@361ad13626117ee715ee1e215458329e8d0bada4.