Sync with OpenBSD changes wrt 64-bit types.

OpenBSD moved from 32-bit time_t to 64-bit time_t (along with several other
type changes). This brings jnr-posix back in sync OpenBSD and effectively
unbreaking it on OpenBSD >= 5.5.
Merge pull request #36 from jasperla/openbsd64

Sync with OpenBSD changes wrt 64-bit types.