Revert "src/usermod.c: Remove optimizations"

Revert "src/usermod.c: Remove optimizations"

This wasn't only an optimization; it also skipped some checks that were
now spuriously triggering errors.  We may be able to get rid of the
optimizations, but that will need more analysis.  For now, let's revert
to a known-good state.

Fixes: 6a8a25dc7de6 (2025-10-15; "src/usermod.c: Remove optimizations")
Reverts: 6a8a25dc7de6 (2025-10-15; "src/usermod.c: Remove optimizations")
Closes: <https://github.com/shadow-maint/shadow/issues/1509>
Reported-by: Adam Williamson <awilliam@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Cherry-picked-from: d25bbc48d607 (2026-01-19; "Revert "src/usermod.c: Remove optimizations"")
Signed-off-by: Alejandro Colomar <alx@kernel.org>