OpenBSD and NetBSD provide macros to directly reference the underlying
struct timespec's tv_nsec member. While FreeBSD has such macros for
tv_sec, the others are missing. The following macros are added:

st->st_atimensec
st->st_mtimensec
st->st_ctimensec
st->st_birthtimensec

Adding these fields will provide programs which reference them better
portability to FreeBSD. An example of such a program is makefs(8), which
has unused support for subseconds that it has inherited from NetBSD.