Synchronize <sys/time.h> with FreeBSD

Synchronize <sys/time.h> with FreeBSD

This change is based on the FreeBSD commit:

Author: asomers <asomers@FreeBSD.org>
Date:   Mon Jul 30 15:46:40 2018 +0000

    Make timespecadd(3) and friends public

    The timespecadd(3) family of macros were imported from NetBSD back in
    r35029. However, they were initially guarded by #ifdef _KERNEL. In the
    meantime, we have grown at least 28 syscalls that use timespecs in some
    way, leading many programs both inside and outside of the base system to
    redefine those macros. It's better just to make the definitions public.

    Our kernel currently defines two-argument versions of timespecadd and
    timespecsub.  NetBSD, OpenBSD, and FreeDesktop.org's libbsd, however, define
    three-argument versions.  Solaris also defines a three-argument version, but
    only in its kernel.  This revision changes our definition to match the
    common three-argument version.

    Bump _FreeBSD_version due to the breaking KPI change.

    Discussed with: cem, jilles, ian, bde
    Differential Revision:  https://reviews.freebsd.org/D14725