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

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