doc/3dformat.htm,src/img.c: The prefix is often unchanged from one
leg to the next, so use a spare flag to compactly indicate that
there's no label change.
src/img.c: Mask out the bit used to indicate no label change from
the returned leg flags.
doc/3dformat.htm,src/img.c: Move the DATE and ERROR codes in 3d v8 to
leave a large contiguous block of codes available.
lib/de.po: Merge German translation updates from launchpad.