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

usr.bin/truss/syscalls.c

@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/socket.h>
 #define _WANT_FREEBSD11_STAT
 #include <sys/stat.h>
+#include <sys/time.h>
 #include <sys/un.h>
 #include <sys/wait.h>
 #include <netinet/in.h>
@@ -2639,7 +2640,7 @@ print_syscall_ret(struct trussinfo *trussinfo, int errorp, long *retval)
 	t = trussinfo->curthread;
 	sc = t->cs.sc;
 	if (trussinfo->flags & COUNTONLY) {
-		timespecsubt(&t->after, &t->before, &timediff);
+		timespecsub(&t->after, &t->before, &timediff);
 		timespecadd(&sc->time, &timediff, &sc->time);
 		sc->ncalls++;
 		if (errorp)