pgsql: Fix plpgsql named-cursor-parameter feature for variable name con

Fix plpgsql named-cursor-parameter feature for variable name conflicts.

The parser got confused if a cursor parameter had the same name as
a plpgsql variable. Reported and diagnosed by Yeb Havinga, though
this isn't exactly his proposed fix.

Also, some mostly-but-not-entirely-cosmetic adjustments to the original
named-cursor-parameter patch, for code readability and better error
diagnostics.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/05dbd4a7734e09bd1f835f4197d9befa1c00c4f3

Modified Files
--------------
src/pl/plpgsql/src/gram.y | 39 ++++++++++++++++++--------------
src/test/regress/expected/plpgsql.out | 19 ++++++++++++++++
src/test/regress/sql/plpgsql.sql | 15 ++++++++++++
3 files changed, 56 insertions(+), 17 deletions(-)