Normalize stat(2) timespecs in _helpers.c.

Previously, the code required timespecs to have a nanosecond value
between 0 and 999999999, because that's what Linux appeared to
produce, but other platforms don't (i.e. Cygwin) -- so normalize the
system values to match expectations.

In particular, look for negative nanosecond values (must be greater
than -10**9), and if found, rewrite the timespec with non-negative
nanoseconds (still less than 10**9).

Thanks to Will Rouesnel <w.rouesnel@gmail.com> for reporting the
problem and posting an initial patch.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>