net/rpc: avoid racy use of closing flag.

It's accessed without mutex protection
in a different goroutine from the one that
sets it.

Also make sure that Client.Call after Client.Close
will reliably return ErrShutdown, and that clients
see ErrShutdown rather than io.EOF when appropriate.

Suggestions welcome for a way to reliably test
the mutex issue.

R=r, iant
CC=golang-dev
https://codereview.appspot.com/7338045
go/parser: more precise comment

See also CL 7383051 for details.

R=adonovan, bradfitz
CC=golang-dev
https://codereview.appspot.com/7378063