D8402 adds more functionality to curve bevels, and this section of code was getting long and messy already,
so now is a good time to separate it from curve.c (which is already quite long).

This patch also:

1. Adds fill_type enum, just local to curve_bevel.c. This makes testing which sections of the curve to build much more intuitive.
2. Uses use_bevel and use_extrude shorthand variables.
3. Splits each case into a separate function, so the main BKE_curve_bevel_make function is more readable.