thread.Halt: Remove duplication between OS implementations

Bring similar code up to top level, and keep OS dependant code in OS
dependant files. DRY up code a bit.

proc/threads.go

@@ -299,3 +299,19 @@ func (thread *Thread) GetG() (g *G, err error) {
 func (thread *Thread) Stopped() bool {
 	return thread.stopped()
 }
+
+// Stops this thread from executing. Actual
+// implementation is OS dependant. Look in OS
+// thread file.
+func (thread *Thread) Halt() (err error) {
+	defer func() {
+		if err == nil {
+			thread.running = false
+		}
+	}()
+	if thread.Stopped() {
+		return
+	}
+	err = thread.halt()
+	return
+}

proc/threads_darwin.go

@@ -12,15 +12,7 @@ type OSSpecificDetails struct {
 	registers  C.x86_thread_state64_t
 }
 
-func (t *Thread) Halt() (err error) {
-	defer func() {
-		if err == nil {
-			t.running = false
-		}
-	}()
-	if t.Stopped() {
-		return
-	}
+func (t *Thread) halt() (err error) {
 	kret := C.thread_suspend(t.os.thread_act)
 	if kret != C.KERN_SUCCESS {
 		errStr := C.GoString(C.mach_error_string(C.mach_error_t(kret)))

proc/threads_linux.go

@@ -12,15 +12,7 @@ type OSSpecificDetails struct {
 	registers sys.PtraceRegs
 }
 
-func (t *Thread) Halt() (err error) {
-	defer func() {
-		if err == nil {
-			t.running = false
-		}
-	}()
-	if t.Stopped() {
-		return
-	}
+func (t *Thread) halt() (err error) {
 	err = sys.Tgkill(t.dbp.Pid, t.Id, sys.SIGSTOP)
 	if err != nil {
 		err = fmt.Errorf("halt err %s on thread %d", err, t.Id)