common: warn when become_user() leaves root credentials

[user01010111]

Aims to close #3471, pending maintainer review and approval.

This makes the shared POSIX become_user() path warn once when a daemon's real or effective UID remains zero after a successful credential transition.

The previous code reported the resulting UID only in a debug-level success message after initgroups(), setgid() and setuid(), and did not warn when the resulting credentials remained root. Checking both resulting UIDs in the shared helper gives daemon callers one consistent invariant without duplicating warnings. Non-root starts retain their existing behaviour, and the intentionally privileged upsmon -p path retains its existing warning without receiving another one.

Validation:

• GCC 16.2.1 built common.o, upslog, upsmon, upsd and dummy-ups successfully. Clang 22.1.8 built the same targets with -Weverything -Werror.
• Before the change, a disposable root namespace running upslog -u root completed with no root warning. The candidate emitted exactly one Warning: running as root (UID=0 EUID=0).
• The equivalent non-root run emitted no root warning. A bounded upsmon -p run retained exactly one existing intentional-root warning and emitted no new shared warning.
• The focused root regression passed, and make -C tests check passed all 9 tests.
• git diff --check, the shell syntax check and make stylecheck passed. A supplementary non-ASCII scan found only an unchanged character outside the diff.
• distcheck-light created and built the clean source archive, including the new regression, then failed at the final spellcheck on unchanged docs/man/apcmicrolink.txt text.
• The behavioural checks cover the POSIX implementation. The WIN32 branch is unchanged, and no physical UPS hardware was required or tested.

AI assistance: OpenAI Codex gpt-5.6-sol at high reasoning was used for investigation, implementation and validation. The human contributor remains responsible for the change.

[github-actions]

A ZIP file with standard source tarball and another tarball with pre-built docs for commit 75d5b30 is temporarily available: NUT-tarballs-PR-3609.zip.

[AppVeyorBot]

✅ Build nut 2.8.5.5181-master completed (commit 3802a9a93c by @)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.5181-master.7z

[jimklimov]

LGTM, but a NEWS.adoc entry would be welcome.

[user01010111]

Thanks Jim — I’ve added a concise NEWS entry under common code. I also corrected the new regression’s build-tree wiring to honour $(EXEEXT) and pass the exact upslog path, and made it use the current UID-0 account name rather than assuming it is root. The focused GCC build/test, UID-0 namespace regression, Automake regeneration and distribution-membership checks pass.

[AppVeyorBot]

✅ Build nut 2.8.5.5186-master completed (commit 117bf537d9 by @)

• artifacts
• A 7-zip archive with built NUT for Windows debug binaries is temporarily available: NUT-for-Windows-x86_64-SNAPSHOT-2.8.5.5186-master.7z

[AppVeyorBot]

✅ Build nut 2.8.5.5186-master completed (commit 117bf537d9 by @)