libfdisk: (gpt) cleanup header size usage

libfdisk: (gpt) cleanup header size usage

Kay reported that Windows 8.1 installed does not recognize GPT created
by libfdisk, but parted works as expected.

It seems (according to the header hexdump) that the problem is
HeaderSize GPT field where libfdisk uses 512 (header + reserved area),
but parted uses 92. The both is pretty valid according to the current
EFI standard, but 92 is probably more backwardly compatible.

I also found places in code where libfdisk gpt driver is inconsistent
and uses sometimes all sector and simetimes sizeof(gpt_header) only.
The right solution is to read all sector (into zeroized memory) and
later write again all sector to the device.

Reported-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Karel Zak <kzak@redhat.com>