pgsql: Allow a multi-row INSERT to specify DEFAULTs for a generated col

Allow a multi-row INSERT to specify DEFAULTs for a generated column.

One can say "INSERT INTO tab(generated_col) VALUES (DEFAULT)" and not
draw an error.  But the equivalent case for a multi-row VALUES list
always threw an error, even if one properly said DEFAULT in each row.
Fix that.  While here, improve the test cases for nearby logic about
OVERRIDING SYSTEM/USER values.

Dean Rasheed

Discussion: https://postgr.es/m/9q0sgcr416t.fsf@gmx.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/17958972fe3bb03454a4b53756b29d65dc285efa

Modified Files
--------------
src/backend/rewrite/rewriteHandler.c    | 242 ++++++++++++++++++++++++++++----
src/test/regress/expected/generated.out |  47 ++++++-
src/test/regress/expected/identity.out  |  56 ++++++++
src/test/regress/sql/generated.sql      |  21 ++-
src/test/regress/sql/identity.sql       |  23 +++
5 files changed, 357 insertions(+), 32 deletions(-)