Clear a process's procfs trace points upon delivery of SIGKILL.

MT5 candidate. (Desired features for 5.3-RELEASE "More truss problems")

sys/kern/kern_sig.c

@@ -1774,6 +1774,16 @@ do_tdsignal(struct thread *td, int sig, sigtarget_t target)
 	if (action == SIG_HOLD &&
 	    !((prop & SA_CONT) && (p->p_flag & P_STOPPED_SIG)))
 		return;
+	/*
+	 * SIGKILL: Remove procfs STOPEVENTs.
+	 */
+	if (sig == SIGKILL) {
+		/* from procfs_ioctl.c: PIOCBIC */
+		p->p_stops = 0;
+		/* from procfs_ioctl.c: PIOCCONT */
+		p->p_step = 0;
+		wakeup(&p->p_step);
+	}
 	/*
 	 * Some signals have a process-wide effect and a per-thread
 	 * component.  Most processing occurs when the process next