- Code refactoring and small optimizations.
- Really fix unwanted acquisition.
- ExternalMethod: fix security declarations.
- Fix role/permission checking for ExternalMethod called by a PythonScript
  with proxy roles.
- When editing an existing guard, modify it instead of always recreate one.
  no very useful here, but that's good practice: this is nicer for the ZODB
  and it's easier to browse the history. BT should do the same when upgrading.