"nm-device.c" is huge, and it does complicated things like handling the
state of the device and IP configuration.
It also contains simpler, individual functions, like converting enums to
strings. Let's move those trivial functions to a new module, so that the
remaining part is smaller.
"nm-device-utils.[ch]" should only contain simpler functions that have
no complex behavior or state.
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/840
