With the current implementation, xml unmarshalling
will silently fail to unmarshal any paths passing
through the same element, such as:

type T struct {
	A string "dummy>a"
	B string "dummy>b"
}

This change tweaks the algorithm so that this works
correctly.

Also, using paths that would cause the same element to
unmarshal twice will error out ahead of time explaining
the problem, rather than silently misbehaving.

R=rsc
CC=golang-dev
https://golang.org/cl/4082041