sys/stat.h: Improve timespec compatibility with other BSDs

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. Add the following macros:

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.

Submitted by: Mitchell Horne <mhorne063@gmail.com>
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D19626