config: support default user on IBM i

[dmabupt]

setuid() can not accept a 'nobody' parameter on IBM i.
The default user (QSECOFR) on IBM i has user id 0.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[addaleax]

Changes to npm should be made upstream to https://github.com/npm/cli, not here.

[dmabupt]

Close this pull request and move it to npm/cli#65
@gireeshpunathil @addaleax
Would you please review npm/cli#65 ?

[gireeshpunathil]

@dmabupt - I am not a collaborator on that repo, so cannot do formal review

[sam-github]

@gireeshpunathil Do you know who at IBM would be able to test that change and confirm to npm that it is correct?

[gireeshpunathil]

I guess @dmabupt himself, if not the i focal is @ThePrez

[ThePrez]

@sam-github, that would be me, @mgoffmn, or @dmabupt (the submitter is from the IBM development team, sorry for any confusion). We have reviewed and tested.