Right now command iteration is not directly controllable by the
user; it is controlled entirely by the application command flag
setup. Sometimes we don't want commands to iterate but only operate
on the currently selected object.
To do this, add a function to supply a "non-iterating" user command
that will execute an iterating-capable command as though it
CMD_FLAG_ONESHOT was set. Add a new command line option to xfs_io to
drive it (-C <command>) and connect it all up. Document it in the
xfs_io man page, too.