gdisk: rename to gptfdisk

In order to match the new upstream name, rename the gdisk package to
gptfdisk. We add the relevant legacy configuration options to ensure a
smooth transition for users.

[Peter: fixup Config.in indentation]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>