Does not address the issues that the existing logging is much too verbose
and does not provide necessary context for what it's complaining. The
logging messages should be improved.

At least, the logging macro gives all messages a "ifupdown: " prefix.

We already have a linked-list implementation. Use it.

Don't listen to udev to find out about devices. First of all, using udev
for this is already very wrong, because we have the platform cache.

Anyway, all that the device information is used, is pointless as well.
Drop it.

It's pointless because:

The entires in eni_ifaces are already indexed by the interface name.
Likewise, all NMIfupdownConnection set "connection.interface-name" to
restict the profile by name.
/e/n/i matches devices is by name, that's it. We don't need udev to
look up the MAC address (by name!!) to later ignore/match devices
by MAC address. Especially, because NetworkMaanger can already
restrict profiles to devices based on the interface name.
Likewise, NetworkMaanger can use the interface name for the
unmanaged-specs.
It's wrong to extend the on-disk configuration from /e/n/i with runtime
information from udev, especially, because other NetworkMaanger layers
are perfectly content using the interface name for this purpose.

Also, bind_device_to_connection() was fundamentally wrong. It's wrong
to modify the settings connection at random moments (on udev event).
If at all, that should only happen during connection load/reload.

This may have been necessary a long time ago, when unmanaged devices were
not expressed by device-match-specs, but by the HAL UDI. That was since
improved, for example by commit c9067d8fedf6f6f2d530fd68bbfca7ce68638d38.