fdisk: API: add geometry

fdisk: API: add geometry

Add device geometry to the fdisk API. While it maintains traditional behaviour, the cylinders
are changed to sector_t instead of unsigned int in order to avoid dealing with truncated cylinders.
A new helper is added to compute the amount of cylinders based on the heads and sectors - if a user passed
-H or -S to the program, it must call this function to update the corresponding values.

This patch passes regression tests.

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