size unpacking fix

ssize, alen, and initsize were being read as doubles and gave garbage values.
Reading them in as a long long (<Q) also gave garbage values.

Reading them in as unsigned longs with four pad bits (<Lxxxx) seems to work and give expected values. It's strange to me that there are four pad bits, but this is the implementation I found that seems to work.
Tested by looking at a few files and it gave correct sizes. It's possible that they should be read in as signed longs (<lxxxx) or ints (<Ixxxx). All three interpretations seemed to give the same values, so it shouldn't matter?

Also made the change for start_vcn and end_vcn, but haven't tested for correctness. It's possible that start_vcn and end_vcn should be unpacked as doubles as before, but it seems unlikely.