pkg: Removing duplicated words ("of of", etc.), mostly from comments.

Ran 'double.pl' on the pkg tree to identify doubled words.
One change to an error string return in x509;  the rest are in comments.
Thanks to Matt Jibson for the idea.

R=golang-dev, bsiegert
CC=golang-dev
https://golang.org/cl/6344089

src/pkg/os/exec/exec.go

@@ -16,7 +16,7 @@ import (
 	"syscall"
 )
 
-// Error records the name of a binary that failed to be be executed
+// Error records the name of a binary that failed to be executed
 // and the reason it failed.
 type Error struct {
 	Name string