spec: pointer comparison for pointers to 0-sized variables
spec: pointer comparison for pointers to 0-sized variables

- define "0-sized"
- add clarifying sentence to pointer comparison
- removed notion "location" which was used only in pointer comparisons
  and which was never defined

Fixes issue 2620.

R=r, rsc, iant
CC=golang-dev
http://codereview.appspot.com/5528053

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