users: add utmp fallback

[mattsu2020]

Summary

• Gate all feat_systemd_logind #[cfg] attributes with target_os = "linux" so that non-Linux builds (macOS, FreeBSD, etc.) are unaffected by the libsystemd link requirement
• Fall back to traditional getutxent() when SystemdUtmpxIter::new() fails at runtime, instead of returning an empty iterator

Context

On Ubuntu 26.04, users prints nothing while GNU users correctly shows logged-in users. Modern systemd no longer populates /var/run/utmp, so utilities that rely on it produce no output. The feat_systemd_logind feature was opt-in, meaning default builds never queried systemd-logind.

[oech3]

Is this dynamically linked againsyt libsystemd.so? If so, this should be disabled by default.

[mattsu2020]

Is this dynamically linked againsyt libsystemd.so? If so, this should be disabled by default.

Change to fallback only

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)