os: release process handle at the end of windows (*Process).Wait

Fixes #3154.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5707052
diff --git a/src/pkg/os/exec_unix.go b/src/pkg/os/exec_unix.go
index 8d000e9..3f89fe8 100644
--- a/src/pkg/os/exec_unix.go
+++ b/src/pkg/os/exec_unix.go
@@ -51,8 +51,7 @@
 	return nil
 }
 
-// Release releases any resources associated with the Process.
-func (p *Process) Release() error {
+func (p *Process) release() error {
 	// NOOP for unix.
 	p.Pid = -1
 	// no need for a finalizer anymore