discovery: Expose discovery cycle time
spanning_tree: Options for startup behavior

This patch introduces two options for startup behavior:
--no-flood: Disables flooding on all ports as soon as a switch
  connects.  This is mostly courtesy of Alex Silva and
  closes #52.
--hold-down: Prohibits changing of the no_flood bit until a
  complete discovery cycle has completed.  Probably mostly
  useful in conjunction with --no-flood.