Solving "device not managed" for Ethernet connections in Network Manager

At some point or another, Network Manager in Debian sid stopped managing my Ethernet connection. The source of the issue is /etc/NetworkManager/NetworkManager.conf:

[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=false

The [ifupdown] clause seems to be telling NM not to do it. The solution is to turn the frown upside down, but doing so in NetworkManager.conf means the setting will likely be clobbered the next time NM is updated. The solution to that is to create a file in in conf.d named 10-globally-managed-devices.conf. NM upgrades should leave custom files in conf.d alone.

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