os/exec: allow simultaneous cmd.Wait and Write of cmd.StdinPipe

cmd.StdinPipe returns an io.WriteCloser.
It's reasonable to expect the caller not to call Write and Close simultaneously,
but there is an implicit Close in cmd.Wait that's not obvious.
We already synchronize the implicit Close in cmd.Wait against
any explicit Close from the caller. Also synchronize that implicit
Close against any explicit Write from the caller.

Fixes #9307.

Change-Id: I8561e9369d6e5ac88dfbca1175549f6dfa04b8ac
Reviewed-on: https://go-review.googlesource.com/31148


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>