os.Error API: don't export os.ErrorString, use os.NewError consistently

This is a core API change.

1) gofix misc src
2) Manual adjustments to the following files under src/pkg:
   gob/decode.go
   rpc/client.go
   os/error.go
   io/io.go
   bufio/bufio.go
   http/request.go
   websocket/client.go
as well as:
   src/cmd/gofix/testdata/*.go.in (reverted)
   test/fixedbugs/bug243.go
3) Implemented gofix patch (oserrorstring.go) and test case (oserrorstring_test.go)

Compiles and runs all tests.

R=r, rsc, gri
CC=golang-dev
https://golang.org/cl/4607052
diff --git a/src/pkg/bytes/buffer.go b/src/pkg/bytes/buffer.go
index 1acd4e0..5de8610 100644
--- a/src/pkg/bytes/buffer.go
+++ b/src/pkg/bytes/buffer.go
@@ -280,7 +280,7 @@
 // from any read operation.)
 func (b *Buffer) UnreadRune() os.Error {
 	if b.lastRead != opReadRune {
-		return os.ErrorString("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
+		return os.NewError("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
 	}
 	b.lastRead = opInvalid
 	if b.off > 0 {
@@ -295,7 +295,7 @@
 // returns an error.
 func (b *Buffer) UnreadByte() os.Error {
 	if b.lastRead != opReadRune && b.lastRead != opRead {
-		return os.ErrorString("bytes.Buffer: UnreadByte: previous operation was not a read")
+		return os.NewError("bytes.Buffer: UnreadByte: previous operation was not a read")
 	}
 	b.lastRead = opInvalid
 	if b.off > 0 {