Remove the possible null value for bitcoind.prune and set prune = 0 in
bitcoind as a default. Remove prune = 0 in secure-node.nix and the
mkForce in configuration.nix (bitcoind.prune = lib.mkForce ).
Instead of forking this repo, it is now recommended that users simply import the
nix-bitcoin module. This commit adds an example directory that contains the
network/ examples and a shell.nix for deployment with nixops.