flag: document the zero value of FlagSet

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12403043

src/pkg/flag/flag.go

@@ -256,7 +256,8 @@ const (
 	PanicOnError
 )
 
-// A FlagSet represents a set of defined flags.
+// A FlagSet represents a set of defined flags.  The zero value of a FlagSet
+// has no name and has ContinueOnError error handling.
 type FlagSet struct {
 	// Usage is the function called when an error occurs while parsing flags.
 	// The field is a function (not a method) that may be changed to point to
@@ -391,7 +392,11 @@ func PrintDefaults() {
 
 // defaultUsage is the default function to print a usage message.
 func defaultUsage(f *FlagSet) {
-	fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
+	if f.name == "" {
+		fmt.Fprintf(f.out(), "Usage:\n")
+	} else {
+		fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
+	}
 	f.PrintDefaults()
 }
 
@@ -658,7 +663,12 @@ func (f *FlagSet) Var(value Value, name string, usage string) {
 	flag := &Flag{name, usage, value, value.String()}
 	_, alreadythere := f.formal[name]
 	if alreadythere {
-		msg := fmt.Sprintf("%s flag redefined: %s", f.name, name)
+		var msg string
+		if f.name == "" {
+			msg = fmt.Sprintf("flag redefined: %s", name)
+		} else {
+			msg = fmt.Sprintf("%s flag redefined: %s", f.name, name)
+		}
 		fmt.Fprintln(f.out(), msg)
 		panic(msg) // Happens only if flags are declared with identical names
 	}