systemd-udevd: limit children-max by available memory (#8668)

systemd-udevd: limit children-max by available memory (#8668)

Udev workers consume typically 50-100MiB virtual memory.
On systems with lots of CPUs and relatively low memory, that may
easily cause workers to be OOM-killed.

This patch limits the number of workers to 8 per GiB memory.
But don't let the limit drop below the smallest value we had
without this patch (8 + 1 * 2 = 10); on small systems, udev's
memory footprint is likely lower.