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 issue 6071

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/13693043
doc/go1.2.html: net/http, encoding, performance, library changes
Everything is done except compiler changes and additional platforms, if any.

R=golang-dev, dominik.honnef, adg
CC=golang-dev
https://codereview.appspot.com/13484045