fdisk: API: add mbr

fdisk: API: add mbr

This patch adds to the fdisk API the relevant logic to buffers that have MBR. This also serves for
future GPT support for the protective MBR. All labels have been updated to have access to the cxt
structure for the corresponding buffer.

An important observation is that SGI no longer uses the standard qsort(3) function to sort partitions,
as it needs access to cxt. To address this, a heap sort implementation from the kernel was added to
the label code and adapted to fdisk.

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