This adds a new sculpt mode to the experimental new (hair) curves object. Currently, this mode can only be entered and existed, nothing else.
The patch also adds the editors/curves/ directory for the new curves object, which will be necessary for many other things as well.
Note, I added a completely new mode (OB_MODE_SCULPT_CURVES), because OB_MODE_SCULPT seems to be rather specific to meshes, and reusing it doesn't seem worth the trouble (not sure what the benefit would be anyway, grease pencil uses OB_MODE_SCULPT_GPENCIL). For the new edit mode, we should probably reuse OB_MODE_EDIT, because it is not specific to any object type.
I had to add DNA_userdef_enums.h to make the patch compile with C++ (forward declaration of enums isn't allowed). This follows the same pattern that we use for other enums in dna.