file: use size_t for position and pointer

file: use size_t for position and pointer

The bufsz variable is used to store the size of the buf memory region
and pos is used to index a position in this memory. Use size_t for these
variables in the internal handling instaed of int to not break with big
files.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>