Fix a bug where marginally old files couldn't be stored in the index

Fix a bug where marginally old files couldn't be stored in the index

Due to the struct having unsigned timestamps, files with dates between Dec 13,
1901 and Jan 1, 1970 were not representable.  This change extends the struct to
be able to pack signed timestamps, which was the spirit of code in _fixup, and
extends the useful range back to 1901. Timestamps prior to 1901 are still
adjusted to zero, as they were before.

There should be no compatibility problems loading packed structures created
before this change, since positive values were truncated at 0x7fffffff.

Signed-off-by: Tim Hatch <tim@timhatch.com>