pgsql: Be more clear when a new column name collides with a system colu

Be more clear when a new column name collides with a system column name.

We now use the same error message for ALTER TABLE .. ADD COLUMN or
ALTER TABLE .. RENAME COLUMN that we do for CREATE TABLE.  The old
message was accurate, but might be confusing to users not aware of our
system columns.

Vik Reykja, with some changes by me, and further proofreading by Tom Lane

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2d1371d3ee5cf7e96d16fb503c27e060df9497f7

Modified Files
--------------
src/backend/commands/tablecmds.c          |   64 ++++++++++++++++++++---------
src/test/regress/expected/alter_table.out |    2 +
src/test/regress/expected/errors.out      |    2 +-
src/test/regress/sql/alter_table.sql      |    2 +
4 files changed, 49 insertions(+), 21 deletions(-)