clarified signature: allow compound arg;

--- a/src/Pure/System/isabelle_system.scala	Thu Nov 14 13:20:09 2019 +0100
+++ b/src/Pure/System/isabelle_system.scala	Thu Nov 14 13:40:21 2019 +0100
@@ -152,10 +152,10 @@
   /* permissions */
 
   def chmod(arg: String, path: Path): Unit =
-    bash("chmod " + Bash.string(arg) + " " + File.bash_path(path)).check
+    bash("chmod " + arg + " " + File.bash_path(path)).check
 
   def chown(arg: String, path: Path): Unit =
-    bash("chown " + Bash.string(arg) + " " + File.bash_path(path)).check
+    bash("chown " + arg + " " + File.bash_path(path)).check
 
 
   /* directories */