Implement IS NOT DISTINCT FROM, update the regression tests and docs.

Neil Conway <neilc@samurai.com>

Commit: be8100d64ec93ccd8160b37379ba189aab4d0ef1
Author: Neil Conway <neilc@samurai.com>
Date: 2005-12-11T10:54:28Z
Releases: 8.2.0
Implement IS NOT DISTINCT FROM, update the regression tests and docs.
Patch from Pavel Stehule, minor fixups by myself.

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