pcu: Support Neighbor Address Resolution over PCUIF

New versions of osmo-pcu support Neighbor Address Resolution over this
interface, and deprecated the old CTRL interface.
Let's use it by default while still keeping support for the old one for
a while to avoid breakage of existing deployments.

Tests run to validate:
* Old osmo-pcu still passes tests using CTRL interface
* New osmo-pcu (with new iface support) still passes tests using CTRL
  interface
* New osmo-pcu (with new iface support) passes tests using new iface

Related: SYS#4971
Change-Id: I05f1aabc64fc5bc4740b0d8afd8990b485eacd50
4 files changed