nixos/systemd: Allow to override serviceConfig

This has been reported by @qknight in his Stack Overflow question:

https://stackoverflow.com/q/50678639

The correct way to override a single value would be to use something
like this:

systemd.services.nagios.serviceConfig.Restart = lib.mkForce "no";

However, this doesn't work because the check is applied for the attrsOf
type and thus the attribute values might still contain the attribute set
created by mkOverride.

The unitOption type however did already account for this, but at this
stage it's already too late.

So now the actual value is unpacked while checking the values of the
attribute set, which should allow us to override values in
serviceConfig.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra, @qknight
(cherry picked from commit 0e7c945)
Reason: Another user has hit this problem on Discourse[1] and I thought
        I had already backported it to 18.03, apparently I didn't. Given
        the time of the original commit I think this had enough testing
        already so it shouldn't break anything and rather make things
        less annoying.
[1]: https://discourse.nixos.org/t/is-there-a-universal-way-to-enable-a-service-auto-restart/592/3

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

@@ -77,10 +77,16 @@ rec {
     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;
+  checkUnitConfig = group: checks: attrs: let
+    # We're applied at the top-level type (attrsOf unitOption), so the actual
+    # unit options might contain attributes from mkOverride that we need to
+    # convert into single values before checking them.
+    defs = mapAttrs (const (v:
+      if v._type or "" == "override" then v.content else v
+    )) attrs;
+    errors = concatMap (c: c group defs) checks;
+  in if errors == [] then true
+     else builtins.trace (concatStringsSep "\n" errors) false;
 
   toOption = x:
     if x == true then "true"