Restic: made modular backup declerations.

Required tree-wide re-wiring of the host option.
Now, rather than each host having a monolithic restic.nix file,
the hosts restic.nix file just specifies the password and url of the
restic repository. Eatch module then definies specific paths to backup
and any pre and post commands that need to be performed.
Each backed up service gets an independent systemd backup service and
timer.

lib/default.nix

@@ -1,6 +1,4 @@
-{lib}: let
-  net = import ./net.nix {inherit lib;};
-in {
+{lib}: {
   dirToStrings = dir: (map (v: builtins.readFile "${dir}/${v}")
     (builtins.filter (v:
       (builtins.readFileType "${dir}/${v}") == "regular") (
@@ -11,13 +9,4 @@ in {
         )
       else []
     )));
-
-  calcSystemdDhcpPoolOffset = {
-    base,
-    start,
-    end,
-  }: {
-    offset = net.lib.net.ip.diff start base;
-    size = net.lib.net.ip.diff end start;
-  };
 }