udev: net_setup - allow matching on OriginalName=

This has been requested repeatedly, so let's give it a go. We explicitly do not allow matching
on names that have already been changed (from a previous udev run, or otherwise), and matching
on unpredictable names (ethX) is discouraged (but not currently disallowed).

We also currently allow:

[Match]
Name=veth0

[Link]
Name=my-name0
SomeOtherSetting=true

Which means that the link file will be applied the first time it is invoked, but
not on subsequent invocations, which may be surprising.