Add control knobs for plpgsql's variable resolution behavior, and make the

Add control knobs for plpgsql's variable resolution behavior, and make the
default be "throw error on conflict", as per discussions.  The GUC variable
is plpgsql.variable_conflict, with values "error", "use_variable",
"use_column".  The behavior can also be specified per-function by inserting
one of
	#variable_conflict error
	#variable_conflict use_variable
	#variable_conflict use_column
at the start of the function body.

The 8.5 release notes will need to mention using "use_variable" to retain
backward-compatible behavior, although we should encourage people to migrate
to the much less mistake-prone "error" setting.

Update the plpgsql documentation to match this and other recent changes.