Added rules to releng/TODO, new options -keep, -clean_up, deprecated -clean

releng/TODO

@@ -1,11 +1,24 @@
 * write a 'master' script to run all tests
- + to be designed & written.
+ -> work in progress: run_all
  + figure out how to store logs and search for interesting
    err/warn keywords spewed up by the script or apps it calls.
- + write common configuration ficility
+ + write common configuration facility
 
 * releng_build_isojigdo:
  + finish boot options
 
 * releng_check_isocontent: compare input data against the ISO image content, might be tricky
- + to be designed & written.
+ -> work in progress
+
+* Give up existing standard option -clean in favor of:
+  + Option -keep which keeps the test data from being removed after
+    the test.
+  + Option -clean_up which removes the test data without doing another test.
+  + Rule: A test must not overwrite existing data on disk.
+          It rather shall refuse and offer option -clean_up to
+          solve the problem (after skilled examination by the user).
+  + Rule: A test shall remove all files which it has created.
+          Only explicit user orders shall allow test files to persist.
+          These orders may be program arguments or configuration variables.
+  -> see releng/test_hardlinks for an implementation example
+