fifo: Require that fifos do not exist for create

Although this operation is safe on linux, it is not safe on Windows when
using the named pipe interface. To provide a ~reasonable common api
abstraction, here we switch to returning File exists errors on the unix
api.

client/lib/fifo/fifo_unix.go

@@ -11,18 +11,18 @@ import (
 )
 
 // CreateAndRead creates a fifo at the given path, and returns an open function for reading.
-// The fifo must not exist already, or that it's already a fifo file
+// For compatibility with windows, the fifo must not exist already.
 //
 // It returns a reader open function that may block until a writer opens
 // so it's advised to run it in a goroutine different from reader goroutine
 func CreateAndRead(path string) (func() (io.ReadCloser, error), error) {
 	// create first
-	if err := mkfifo(path, 0600); err != nil && !os.IsExist(err) {
+	if err := mkfifo(path, 0600); err != nil {
 		return nil, fmt.Errorf("error creating fifo %v: %v", path, err)
 	}
 
 	return func() (io.ReadCloser, error) {
-		return os.OpenFile(path, unix.O_RDONLY, os.ModeNamedPipe)
+		return OpenReader(path)
 	}, nil
 }