PR 13128: Implement the new --privilege=stapsys

PR 13128: Implement the new --privilege=stapsys

- Users belonging to the group stapsys can use all probe points and tapset
  functions (including those which contain embedded C), but cannot
  use the -g option.
- Loading of modules for stapsys users requires signing by a trusted compile-server.
- Add testing for stapsys throughout as appropriate.
- Update existing unprivileged test suites to test stapsys as appropriate.
- New test suite tests -g against all privilege levels.