accept attr args to pppoe service, and typecheck them

We use (abuse, arguably) the nixos module system for typechecking.  Un
the plus side, it gives us documentation of the options and their
expected types. On the downside, the error message doesn't tell us
the file in which the error was encountered.

(This is subject to change, if I can find a better way)

modules/ppp.nix

@@ -6,7 +6,7 @@ let
 in {
   options = {
     system.service.pppoe = mkOption {
-      type = types.functionTo (types.functionTo types.package);
+      type = types.functionTo types.package;
     };
   };
   config = {