sysctl: set ipv4 settings in a race-free way

Fixes #6282.

This solution is a bit busy, but we close the race without setting *.all.*, so
it is still possible to set a different setting for particular interfaces.
Setting just "default" is not very useful because any interfaces present before
systemd-sysctl is invoked are not affected. Setting "all" is too harsh, because
the kernel takes the stronger of the device-specific setting and the "all" value,
so effectively having a weaker setting for specific interfaces is not possible.