pgsql: Re-add default_with_oids GUC to avoid breaking old dump files.

Re-add default_with_oids GUC to avoid breaking old dump files.

After 578b229718 / the removal of WITH OIDS support, older dump files
containing
SET default_with_oids = false;
either report unnecessary errors (as the subsequent tables have no
oids) or even fail to restore entirely (when using transaction mode).
To avoid that, re-add the GUC, but don't allow setting it to true.

Per complaint from Tom Lane.

Author: Amit Khandekar, editorialized by me
Discussion: https://postgr.es/m/CAJ3gD9dZyxrtL0rJfoNoOj6v7fJSDaXBngi9wy5XU8m-ioXhAA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/de66987adb8a3414724b99540d324407bfad697a

Modified Files
--------------
src/backend/utils/misc/guc.c | 37 +++++++++++++++++++++++++++++++++++++
src/test/regress/expected/guc.out | 4 ++++
src/test/regress/sql/guc.sql | 4 ++++
3 files changed, 45 insertions(+)