nixos/modules: Remove all usages of types.string

And replace them with a more appropriate type

Also fix up some minor module problems along the way

nixos/modules/services/logging/SystemdJournal2Gelf.nix

@@ -16,7 +16,7 @@ in
       };
 
       graylogServer = mkOption {
-        type = types.string;
+        type = types.str;
         example = "graylog2.example.com:11201";
         description = ''
           Host and port of your graylog2 input. This should be a GELF
@@ -25,7 +25,7 @@ in
       };
 
       extraOptions = mkOption {
-        type = types.string;
+        type = types.separatedString " ";
         default = "";
         description = ''
           Any extra flags to pass to SystemdJournal2Gelf. Note that
@@ -56,4 +56,4 @@ in
       };
     };
   };
-}
+}