Merge changes from Go's standard flag library.

Up to date through e7cd0a82d669. Relevant changesets:

https://code.google.com/p/go/source/detail?r=059ba0d0dc6a27d115c619ff8bcd646335eed67f
https://code.google.com/p/go/source/detail?r=64a249c65d510184268160bf595cf3db478a296e
https://code.google.com/p/go/source/detail?r=6ca4825bc86760d43392469c0af266bb858fc77b

README.md

diff --git a/README.md b/README.md
index 714d202..a9506fd 100644
--- a/README.md
+++ b/README.md
@@ -40,7 +40,9 @@
 functions such as String(), BoolVar(), and Var(), and is therefore
 unaffected.
 
-Define flags using flag.String(), Bool(), Int(), etc. Example:
+Define flags using flag.String(), Bool(), Int(), etc.
+
+This declares an integer flag, -flagname, stored in the pointer ip, with type *int.
 
 ``` go
 var ip *int = flag.Int("flagname", 1234, "help message for flagname")
@@ -76,13 +78,13 @@
 they are all pointers; if you bind to variables, they're values.
 
 ``` go
-fmt.Println("ip has value ", *ip);
-fmt.Println("flagvar has value ", flagvar);
+fmt.Println("ip has value ", *ip)
+fmt.Println("flagvar has value ", flagvar)
 ```
 
 After parsing, the arguments after the flag are available as the
 slice flag.Args() or individually as flag.Arg(i).
-The arguments are indexed from 0 up to flag.NArg().
+The arguments are indexed from 0 through flag.NArg()-1.
 
 The pflag package also defines some new functions that are not in flag,
 that give one-letter shorthands for flags. You can use these by appending

flag.go

diff --git a/flag.go b/flag.go
index 06edf20..a698e27 100644
--- a/flag.go
+++ b/flag.go
@@ -24,8 +24,10 @@
 	functions such as String(), BoolVar(), and Var(), and is therefore
 	unaffected.
 
-	Define flags using flag.String(), Bool(), Int(), etc. Example:
-		var ip *int = flag.Int("flagname", 1234, "help message for flagname")
+	Define flags using flag.String(), Bool(), Int(), etc.
+
+	This declares an integer flag, -flagname, stored in the pointer ip, with type *int.
+		var ip = flag.Int("flagname", 1234, "help message for flagname")
 	If you like, you can bind the flag to a variable using the Var() functions.
 		var flagvar int
 		func init() {
@@ -42,12 +44,12 @@
 
 	Flags may then be used directly. If you're using the flags themselves,
 	they are all pointers; if you bind to variables, they're values.
-		fmt.Println("ip has value ", *ip);
-		fmt.Println("flagvar has value ", flagvar);
+		fmt.Println("ip has value ", *ip)
+		fmt.Println("flagvar has value ", flagvar)
 
 	After parsing, the arguments after the flag are available as the
 	slice flag.Args() or individually as flag.Arg(i).
-	The arguments are indexed from 0 up to flag.NArg().
+	The arguments are indexed from 0 through flag.NArg()-1.
 
 	The pflag package also defines some new functions that are not in flag,
 	that give one-letter shorthands for flags. You can use these by appending
@@ -843,8 +845,9 @@
 	flag := &Flag{name, shorthand, usage, value, value.String()}
 	_, alreadythere := f.formal[name]
 	if alreadythere {
-		fmt.Fprintf(f.out(), "%s flag redefined: %s\n", f.name, name)
-		panic("flag redefinition") // Happens only if flags are declared with identical names
+		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
 	}
 	if f.formal == nil {
 		f.formal = make(map[string]*Flag)