pgsql: Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

Remove arbitrary ALTER TABLE .. ADD COLUMN restriction.

The previous coding prevented ALTER TABLE .. ADD COLUMN from being used
with a non-NULL default in situations where the table's rowtype was being
used elsewhere. But this is a completely arbitrary restriction since
you could do the same operation in multiple steps (add the column, add
the default, update the table).

Inspired by a patch from Noah Misch, though I didn't use his code.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=a06e41deebdf74b8b5109329dc75b2e9d9057962

Modified Files
--------------
src/backend/commands/tablecmds.c | 8 +++++---
src/test/regress/expected/rowtypes.out | 9 ++++++---
src/test/regress/sql/rowtypes.sql | 8 ++++++--
3 files changed, 17 insertions(+), 8 deletions(-)