spawn-unix: On Linux, don't try to increase OOM-killer protection

The oom_score_adj parameter is a signed integer, with increasingly
positive values being more likely to be killed by the OOM-killer,
and increasingly negative values being less likely.

Previously, we assumed that oom_score_adj would be negative or zero,
and reset it to zero, which does not require privileges because it
meant we're voluntarily giving up our OOM-killer protection.
In particular, bus/dbus.service.in has OOMScoreAdjust=-900, which
we don't want system services to inherit.

However, systemd >= 250 has started putting a positive oom_score_adj
on user processes, to make it more likely that the OOM killer will kill
a user process rather than a system process. Changing from a positive
oom_score_adj to zero is increasing protection from the OOM-killer,
which only a privileged process is allowed to do, resulting in warnings
whenever we carry out traditional (non-systemd) service activation
on the session bus.

To avoid this, do the equivalent of:

    if (oom_score_adj < 0)
        oom_score_adj = 0;

which is always allowed.

Resolves: https://gitlab.freedesktop.org/dbus/dbus/-/issues/374
Signed-off-by: Simon McVittie <smcv@collabora.com>

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