bug#19135: mklabel gpt always creates 128 partition entries

This is halfway a bug and halfway an enhancement:

parted's "mkpart gpt" always creates a Partition Entry Array with 128 entries.
It may be due to misunderstanding the UEFI docs that state the size of the partition entry should be a multiple of 128.
At the moment the Partition Entry Table needs 16kB (128*128). That's no much, especially when considering that most systems align data structures to 1MB boundaries.
An argument against it would be that at least 90% of all disks have less than 5 partitions, and I guess 99% have less than 10 partitions on the disk, so 128 seems to be an overkill. Even if unused, a program has to read and check all these unused entries.

(I created a GPT label on a 3MB test image)

Part of the enhancement is: Supply an additional parameter to "mklabel gpt" that allows to specify the size (number of entries) in the GPT Partition Entry Table, or even provide a command to resize it.