client: return strictly io.EOF at EOF
client: return strictly io.EOF at EOF

Otherwise this breaks routines like ioutil.ReadAll

Change-Id: Ic4e75d7e4a600b5a408c767ffd64aa5997d8e18b
Reviewed-on: https://upspin-review.git.corp.google.com/2543
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin