This makes properties consistent even after a COPY, MOVE or DELETE.

A follow-up CL will eliminate the concept of a configurable PropSystem
entirely.

See discussion at
https://groups.google.com/forum/#!topic/golang-dev/2_LiN6sf93A

Litmust test before/after:
<- summary for `props': of 30 tests run: 28 passed, 2 failed. 93.3%
<- summary for `props': of 30 tests run: 29 passed, 1 failed. 96.7%

Change-Id: I9bd09d9fb73e40f87306eaec2679945874af023d
Reviewed-on: https://go-review.googlesource.com/10241Reviewed-by: Nigel Tao <nigeltao@golang.org>