Allow pg_dumpall to work without pg_authid

Hi,

I would like to work on a patch to accommodate restricted environments
(such as AWS RDS Postgres) which don't allow pg_authid access since their
definition of Superuser is just a regular user with extra permissions.

Would you consider a patch to add a flag to work around this restriction,
Or, do you prefer that this be maintained outside core?

I could add a flag such as --avoid-pgauthid (am open to options) that skips
pg_authid and uses pg_user (but essentially resets all User passwords).
Mostly this is better than not being able to get the dump at all.

I have a fork here (a few weeks old):
https://github.com/postgres/postgres/commit/552f4d92fde9518f934447e032b8ffcc945f12d9

Thanks
Robins Tharakan
http://www.thatguyfromdelhi.com/2016/12/custom-pgdumpall-now-works-with-aws.html
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-
robins