Work in progress in getting GPTPartitionHandle working.

* Tried to use EFI::Header class, but there doesn't seem to be an easy
way to actually hit the disk -- which we'll have to do to find out
how large the GPT table is.
* Initialization of GPT disks is now working which is why I added the disk
system add-on to the image. However, there is a caveat, as the backup
header and table aren't written yet.
* Partitions can be deleted.
* Creating partitions does not work yet, but I don't know yet why; in
theory it could already work.

Renamed partitioning system "efi" directory to "gpt".

gpt: early work in progress of a disk system add-on.

* It currently allows to initialize a disk using GPT. However, this then
fails somewhere in the kernel.