fdisk: API: add label probing functionality

fdisk: API: add label probing functionality

This patch sets the initial layout for label specific operations. A
new fdisk_label structure is created that will hold all these ops,
like new, delete, write and probe, among others. For now only probing
is implemented. Once this design is established, a copy of the probed
label will be copied to the main context structure, where calling the
specific functions will save 'disklabel' checks. Debugging support is
added as well.

This patch passes regression tests and manually passes bsd, sun, dos
and sgi labels probes.

Reviewed-by: Petr Uzel <petr.uzel@suse.cz>
Signed-off-by: Davidlohr Bueso <dave@gnu.org>