ogle/program/server: clarify ordering of evaluation when ptraceRun
wraps a closure that 'returns' more than just an error.

I don't think this changes the actual behavior with the gc compiler,
but a different compiler might not optimize the "return foo" part
as a no-op, in:
func f() (foo int, err error) {
        etc
        return foo, <-s.ec
}
and then it's possibly racy whether foo is evaluated before or after
the channel receive.

LGTM=r
R=r
https://golang.org/cl/89750043

program/server/ptrace.go

diff --git a/program/server/ptrace.go b/program/server/ptrace.go
index 9bd3357..989f154 100644
--- a/program/server/ptrace.go
+++ b/program/server/ptrace.go
@@ -30,7 +30,8 @@
 		proc, err1 = os.StartProcess(name, argv, attr)
 		return err1
 	}
-	return proc, <-s.ec
+	err = <-s.ec
+	return
 }
 
 func (s *Server) ptraceCont(pid int, signal int) (err error) {
@@ -102,5 +103,6 @@
 		wpid, err1 = syscall.Wait4(pid, &status, syscall.WALL, nil)
 		return err1
 	}
-	return wpid, status, <-s.ec
+	err = <-s.ec
+	return
 }