backport.pl: Improve documentation and handling of modes.

* tools/dist/backport.pl
  (Mode::AutoCommitApproveds, Mode::Conflicts, Mode::Interactive):
    New enumerators.
  ($MODE): New enum-valued global variable.
  (backport_usage): Document "YES=0 MAY_COMMIT=1" interactive RM mode.
  (handle_entry): Record a todo related to a workflow recommended by the new
    usage message.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606352 13f79535-47bb-0310-9956-ffa450edef68