BUG: core: ensure file handle positions are in sync

The Python3 file handles keep records of current file positions, so that
the raw handle needs to be restored to the original position in order
to not confuse the mechanism.

Fixes gh-4118
ENH: core: switch to off_t and lseek in Python 3 file seeks

This removes dependency on CRT implementation details (whether
fflush/fclose actually seeks the underlying raw file handle or not).

Also, check seek error statuses.
BUG: core: use correct Python format string for npy_off_t
BLD: add off_t size detection to bento build
Merge pull request #4152 from pv/fromfile-fix

BUG: core: ensure file handle positions are in sync