fdisk: move DOS new/add partition code

fdisk: move DOS new/add partition code

Since this is DOS specific logic, it belongs in its own label file.
Additionally, a dos_new_partition() function is created that asks the user for
partition type and then calls the actual dos_add_partition().

This patch passed fdisk regression tests, builds without problems and it was
locally tested against adding and removing DOS partitions.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>