Don't bother to turn off other P_STOPPED bits for SIGKILL, doing

so would cause kernel to produce an unkillable process in some cases,
especially, P_STOPPED_SINGLE has a singling thread, turning off the
bit would mess the state.

sys/kern/kern_sig.c

@@ -1812,7 +1812,7 @@ do_tdsignal(struct thread *td, int sig, sigtarget_t target)
 			 * It will die elsewhere.
 			 * All threads must be restarted.
 			 */
-			p->p_flag &= ~P_STOPPED;
+			p->p_flag &= ~P_STOPPED_SIG;
 			goto runfast;
 		}