Prevent accidental whole-table DELETEs and UPDATEs

In many cases, a DELETE or UPDATE not having a WHERE clause (or having it
with a condition matching all rows in the table) is a sign of some kind of
mistake, leading to accidental data loss, performance issues, producing a
lot of dead tuples, and so on. Recently, this topic was again discussed [1]

Attached is a patch implemented by Andrey Boroding (attached) during our
today's online session [2], containing a rough prototype for two new GUCs:

- prevent_unqualified_deletes
- prevent_unqualified_updates

Both are "false" by default; for superusers, they are not applied.

There is also another implementation of this idea, in the form of an
extension [3], but I think having this in the core would be beneficial to
many users.

Looking forward to your feedback.

[1] https://news.ycombinator.com/item?id=34560332
[2] https://www.youtube.com/watch?v=samLkrC5xQA
[3] https://github.com/eradman/pg-safeupdate