Implement terrain merging in TerrainChannel.
Modify archiver to use terrain merging when loading oars.
This makes displacement AND rotation properly work on terrain when loading oars.
Especially useful when loading legacy region oars into large varregions.
Change new 'load oar' parameters to be hyphenated to be consistant with
existing parameters. ('--forceterrain' becomes '--force-terrain').
The old forms have been kept for downward compatiblity.
Really add the old parameter names to load oar to keep downward compatibiliy