Remember to use fseeko(), not fseek()

Fixes opening Hamamatsu NDPI slides > 2 GB and reading MIRAX PNG slides
with a level > 2 GB on 32-bit systems.

http://lists.andrew.cmu.edu/pipermail/openslide-users/2014-April/000772.html
tifflike: Drop redundant seek
hamamatsu: Always check fseeko() return value