Clear p_stops when doing PT_DETACH.

Without this, if a process was being traced by truss(1), which
uses different p_stops bits than gdb(1), the latter would
misbehave because of the unexpected bits.

Reported by:	jceel
Submitted by:	sef
Sponsored by:	iXsystems, Inc.
MFC after:	2 weeks

sys/kern/sys_process.c

@@ -963,6 +963,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 				CTR1(KTR_PTRACE, "PT_DETACH: pid %d", p->p_pid);
 			p->p_oppid = 0;
 			p->p_flag &= ~(P_TRACED | P_WAITED | P_FOLLOWFORK);
+			p->p_stops = 0;
 
 			/* should we send SIGCHLD? */
 			/* childproc_continued(p); */