systemd: use unit name in PrivateTmp directories

Unit name is used whole in the directory name, so that the unit name
can be easily extracted from it, e.g. "/tmp/systemd-abcd.service-DEDBIF1".

https://bugzilla.redhat.com/show_bug.cgi?id=957439

src/core/execute.c

@@ -1089,7 +1089,7 @@ int exec_spawn(ExecCommand *command,
         free(line);
 
         if (context->private_tmp && !context->tmp_dir && !context->var_tmp_dir) {
-                r = setup_tmpdirs(&context->tmp_dir, &context->var_tmp_dir);
+                r = setup_tmpdirs(unit_id, &context->tmp_dir, &context->var_tmp_dir);
                 if (r < 0)
                         return r;
         }