proc,service,terminal: read defer list

Adds -defer flag to the stack command that decorates the stack traces
by associating each stack frame with its deferred calls.

Reworks proc.next to use this feature instead of using proc.DeferPC,
laying the groundwork to implement #1240.

pkg/proc/variables.go

@@ -489,29 +489,18 @@ func (v *Variable) fieldVariable(name string) *Variable {
 	return nil
 }
 
-// PC of entry to top-most deferred function.
-func (g *G) DeferPC() uint64 {
+// Defer returns the top-most defer of the goroutine.
+func (g *G) Defer() *Defer {
 	if g.variable.Unreadable != nil {
-		return 0
+		return nil
 	}
-	d := g.variable.fieldVariable("_defer").maybeDereference()
-	if d.Addr == 0 {
-		return 0
+	dvar := g.variable.fieldVariable("_defer").maybeDereference()
+	if dvar.Addr == 0 {
+		return nil
 	}
-	d.loadValue(LoadConfig{false, 1, 64, 0, -1})
-	if d.Unreadable != nil {
-		return 0
-	}
-	fnvar := d.fieldVariable("fn").maybeDereference()
-	if fnvar.Addr == 0 {
-		return 0
-	}
-	fnvar.loadValue(LoadConfig{false, 1, 64, 0, -1})
-	if fnvar.Unreadable != nil {
-		return 0
-	}
-	deferPC, _ := constant.Int64Val(fnvar.fieldVariable("fn").Value)
-	return uint64(deferPC)
+	d := &Defer{variable: dvar}
+	d.load()
+	return d
 }
 
 // From $GOROOT/src/runtime/traceback.go:597