nixos/postgresql: fix merging of shared_preload_libraries option

Closes #386804

The issue with coercing to `types.str` is that it's not mergeable, so
any declarations will result in an eval error like this:

    error: The option `interactive.nodes.tmp.services.postgresql.settings.shared_preload_libraries' has conflicting definition values:
    - In `/home/ma27/Projects/nixpkgs-hack/tmp.nix@node-tmp': "foo"
    - In `/home/ma27/Projects/nixpkgs-hack/tmp.nix@node-tmp': "bar2"
    Use `lib.mkForce value` or `lib.mkDefault value` to change the priority on any of these definitions.

Using a mergeable type (`types.comma`, i.e. a string, where all declarations
get joined with a comma as delimiter) fixes the problem.

nixos/modules/services/databases/postgresql.nix

@@ -439,7 +439,7 @@ in
             ]);
             options = {
               shared_preload_libraries = mkOption {
-                type = nullOr (coercedTo (listOf str) (concatStringsSep ", ") str);
+                type = nullOr (coercedTo (listOf str) (concatStringsSep ",") commas);
                 default = null;
                 example = literalExpression ''[ "auto_explain" "anon" ]'';
                 description = ''