src/img.c,src/img.h: Add ability to store a proj string describing
the coordinate system in use in 3d v8 files.
doc/3dformat.htm: Update format doc to mention coordinate system.
src/dump3d.c: If there's a coordinate system specified, report it.
src/netskel.c: If there's an output coordinate system set, write it
to the 3d file.