encoding/gob: ignore chan and func fields of structures

encoding/gob: ignore chan and func fields of structures

Previously, fields of type chan or func caused an error.
Now we just treat them like unexported fields and ignore them.
This makes it easier to guarantee long-term compatibilty since
a substructure from another package cannot break gob
encoding by adding a func or chan field.

Fixes #6071

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13693043