Allow setting the user which we drop permissions to using suid.

[alexmv]

What do these changes do?

We may want a different user than nobody when dropping permissions -- for example, if we intend to connect to a PostgreSQL database which does ident authentication.

Are there changes in behavior for the user?

No behaviour change.

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [ ] tox testenvs have been executed in the following environments:
  • [x] Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-{nocov,cov,diffcov}, qa, docs
  • [ ] Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • [ ] WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • [ ] FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • [ ] Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• [x] Documentation reflects the changes
• [x] Add a news fragment into the NEWS.rst file

[codecov[bot]]

Codecov Report

Merging #370 (bedda90) into master (83168cd) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head bedda90 differs from pull request most recent head 9182c42. Consider uploading reports for the commit 9182c42 to get more accurate results

@@            Coverage Diff            @@
##            master      #370   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1706      1707    +1     
  Branches       310       310           
=========================================
+ Hits          1706      1707    +1     

Impacted Files	Coverage Δ
aiosmtpd/main.py	100.00% <100.00%> (ø)

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[alexmv]

Thanks for the review! I've repushed with your documentation change.

I do have a slight concern that there might be a better way to have the other connections made and then drop privileges to nobody... but I don't think that's necessarily a blocker here.

I don't think there is -- neither root nor nobody can auth, so both making the connections before or after calling suid(nobody) isn't feasible.

[alexmv]

Gentle nudge on this -- anything I can do to help move this forward?