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's definitions of timespecadd and timespecsub are slightly different
than NetBSD's and OpenBSD's. This revision changes our definition to match
Bump _FreeBSD_version due to the breaking KPI change.