fdisk: isolate dos label logic

fdisk: isolate dos label logic

DOS specific logic is currently embedded in the heart of fdisk code. This patch
separates DOS label specific code into its own file, just like the rest of the
labels, leaving a more generic fdisk.c file. Most changes are just moving code
from fdisk.c to fdisk.h and fdiskdoslabel.[c/h].

The only logical modification is calling dos_delete_partition() from
read_extended(), instead of the generic delete_partition.  This is ok since
read extended is only called from a DOS context.

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