link-config: add "keep" policy and use it by default

link-config: add "keep" policy and use it by default

If "keep" policy is specified, and the interface has a name that is
NET_NAME_USER or NET_NAME_RENAMED, we stop processing rules. "keep" should
probably be specified either first or last depending on the preference.

This partially reimplements 55b6530baacf4658a183b15b010a8cf3483fde08, in the
sense that if the "keep" policy is not specified, and if the interface has
a NamingPolicy, it will be renamed, even if it had a name previously.
So this breaks backwards compatibility in this case, but that's more in line
with what users expect.

Closes #9006.