effective_go.html: move and rework the blank identifier section

Also rename the relevant examples and make sure the working one compiles.

R=golang-dev, bradfitz, adg, iant, rsc
CC=golang-dev
https://golang.org/cl/7597043

doc/progs/unused1.go → doc/progs/eff_unused1.go

@@ -5,8 +5,14 @@ package main
 import (
 	"fmt"
 	"io"
+	"log"
+	"os"
 )
 
 func main() {
-	greeting := "hello, world"
+	fd, err := os.Open("test.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	// TODO: use fd.
 }

doc/progs/eff_unused2.go

+// compile
+
+package main
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"os"
+)
+
+var _ = fmt.Printf // For debugging; delete when done.
+var _ io.Reader    // For debugging; delete when done.
+
+func main() {
+	fd, err := os.Open("test.go")
+	if err != nil {
+		log.Fatal(err)
+	}
+	// TODO: use fd.
+	_ = fd
+}

doc/progs/run

@@ -16,6 +16,7 @@ effective_go="
 	eff_bytesize
 	eff_qr
 	eff_sequence
+	eff_unused2
 "
 
 error_handling="

doc/progs/unused2.go

-// compile
-
-package main
-
-import (
-	"fmt"
-	"io"
-)
-
-var _ = fmt.Printf
-var _ io.Reader
-
-func main() {
-	greeting := "hello, world"
-	_ = greeting
-}