itd: Reorganize header definition

Since I conveniently decided to pad file table to 64-byte offset, I have
a lot of extra space for data.
So I made it so first 32 bytes are 'main' header and resto of them are
'secondary' header.
itd: Split header struct

I decided to make that distinction even more clear.
itd: Rename dir 'hitd' to 'itd'

This is proper name. I named header 'hitd.h' to be consistent with hdf.
But, HDF is explicitly named 'hrengin Data Format', while ITD is
explicitly named ITD.