networkd: Fix evaluation of systemd.network units.

During the refactor of the networkd stuff in f8dbe5f, a lot of the
options are now needed by systemd.nix as well as networkd.nix but
weren't moved by that commit as well.

For now, this fixes all networkd VM tests except for the macvlan one and
thus it should fix #7505 for at least DHCP-based configuration.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/system/boot/systemd-lib.nix

@@ -25,6 +25,69 @@ rec {
           ln -s /dev/null $out/${shellEscape name}
         '';
 
+  boolValues = [true false "yes" "no"];
+
+  digits = map toString (range 0 9);
+
+  isByteFormat = s:
+    let
+      l = reverseList (stringToCharacters s);
+      suffix = head l;
+      nums = tail l;
+    in elem suffix (["K" "M" "G" "T"] ++ digits)
+      && all (num: elem num digits) nums;
+
+  assertByteFormat = name: group: attr:
+    optional (attr ? ${name} && ! isByteFormat attr.${name})
+      "Systemd ${group} field `${name}' must be in byte format [0-9]+[KMGT].";
+
+  hexChars = stringToCharacters "0123456789abcdefABCDEF";
+
+  isMacAddress = s: stringLength s == 17
+    && flip all (splitString ":" s) (bytes:
+      all (byte: elem byte hexChars) (stringToCharacters bytes)
+    );
+
+  assertMacAddress = name: group: attr:
+    optional (attr ? ${name} && ! isMacAddress attr.${name})
+      "Systemd ${group} field `${name}' must be a valid mac address.";
+
+
+  assertValueOneOf = name: values: group: attr:
+    optional (attr ? ${name} && !elem attr.${name} values)
+      "Systemd ${group} field `${name}' cannot have value `${attr.${name}}'.";
+
+  assertHasField = name: group: attr:
+    optional (!(attr ? ${name}))
+      "Systemd ${group} field `${name}' must exist.";
+
+  assertRange = name: min: max: group: attr:
+    optional (attr ? ${name} && !(min <= attr.${name} && max >= attr.${name}))
+      "Systemd ${group} field `${name}' is outside the range [${toString min},${toString max}]";
+
+  assertOnlyFields = fields: group: attr:
+    let badFields = filter (name: ! elem name fields) (attrNames attr); in
+    optional (badFields != [ ])
+      "Systemd ${group} has extra fields [${concatStringsSep " " badFields}].";
+
+  checkUnitConfig = group: checks: v:
+    let errors = concatMap (c: c group v) checks; in
+    if errors == [] then true
+      else builtins.trace (concatStringsSep "\n" errors) false;
+
+  toOption = x:
+    if x == true then "true"
+    else if x == false then "false"
+    else toString x;
+
+  attrsToSection = as:
+    concatStrings (concatLists (mapAttrsToList (name: value:
+      map (x: ''
+          ${name}=${toOption x}
+        '')
+        (if isList value then value else [value]))
+        as));
+
   generateUnits = type: units: upstreamUnits: upstreamWants:
     pkgs.runCommand "${type}-units" { preferLocalBuild = true; } ''
       mkdir -p $out