Added more libdxf documentation targets.

Signed-off-by: bert <bert.timmerman@xs4all.nl>
STYLE: added dxf_style_read().

Signed-off-by: bert <bert.timmerman@xs4all.nl>
STYLE: correction of an omission in dxf_style_write().

Signed-off-by: bert <bert.timmerman@xs4all.nl>