VintageNet technology for explicitly shared or isolated Ethernet ports.

Delegates bridge/DHCP/address management to the standard technologies. Keeps the fixed Layer 2 bridge alive when changing its management address, including after a manager restart. This also avoids VintageNetBridge 0.10.3's delbr-before- down teardown bug. Members have no IPv6 autoconfiguration or IPv4 addresses. VintageNet remains the sole owner, including interface hotplug and IP cleanup.