udev: lower the starting children_max -> drop the minimum required RAM from 23mb...

Udev was the limiting factor for us on low-RAM systems.
Given an average RSS of 180kb, 128 workers would require ~23mb of RAM.

Now, please consider what happens when there is only, say, 15mb free.
Udev protects itself from OOM, and the kernel can do nothing but panic.

28 workers * 0.18mb = ~5mb. This change should not affect more powerful
systems much, given that they still get the addition from the amount of RAM.