Add PROC_UNLOCK(p) to PTRACESTOP_SC(p, td, flag). This is necessary

due to a change made in revision 1.284 of sys/kern/kern_sig.c in August
2004 which made ptracestop() return with the process still locked.

Submitted by:	Mauritz Sundell
MFC After:	3 days

sys/sys/ptrace.h

@@ -100,6 +100,7 @@ struct ptrace_lwpinfo {
 	if ((p)->p_flag & P_TRACED && (p)->p_stops & (flag)) {	\
 		PROC_LOCK(p);					\
 		ptracestop((td), SIGTRAP);			\
+		PROC_UNLOCK(p);					\
 	}
 /*
  * The flags below are used for ptrace(2) tracing and have no relation