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.

This uses @jeremyevans's patch from #15 . We (Jeremy and I) propose to merge this patch, and thus not include backwards compat for OpenBSD 5.4 and earlier. These releases will not be supported by OpenBSD in 4 weeks from now so there is little merit for jnr-posix to keep supporting it either.