netchan: graceful handling of closed connection

Currently, when an importer closes the connection, the exporter gives an
error message 'netchan export: error decoding client header:EOF'.  This
change causes the exporter to look for an EOF during the parse of the
header, and silences the log message in that case.

R=r
CC=golang-dev, rog
https://golang.org/cl/4132044

src/pkg/netchan/export.go

@@ -118,7 +118,9 @@ func (client *expClient) run() {
 	for {
 		*hdr = header{}
 		if err := client.decode(hdrValue); err != nil {
-			expLog("error decoding client header:", err)
+			if err != os.EOF {
+				expLog("error decoding client header:", err)
+			}
 			break
 		}
 		switch hdr.PayloadType {