Make foo=null a warning by default.

Folks,

Per a discussion with Andrew Gierth and Vik Fearing, both of whom
helped make this happen, please find attached a patch which makes it
possible to get SQL standard behavior for "= NULL", which is an error.
It's been upgraded to a warning, and can still be downgraded to
silence (off) and MS-SQL-compatible behavior (on).

Best,
David.
-- 
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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