It turns out we have some inconsistency with Enable/Disable.  Plus the API docs are unclear and should be updated once we agree on the actual behavior.

It appears that we don't unmanage software devices when NM gets disabled, presumably because if we unmanaged ones NM created, they would get destroyed-on-delete.

Just brainstorming, but setting Enabled=false should probably (a) deactivate any NM-managed network connections, (b) move all devices to unmanaged, but not delete any NM-created ones and don't deconfigure assumed devices, (c) continue recognizing new devices but never advance them past unmanaged, and (d) don't touch resolv.conf or the routing table or anything

-------------

$ nmcli networking
enabled

$ nmcli networking connectivity 
full

$ brctl show
bridge name     bridge id               STP enabled     interfaces
bridge0         8000.001234567830       no              enp3s0

$ nmcli device 
DEVICE      TYPE      STATE         CONNECTION 
bridge0     bridge    connected     bridge0    
enp3s0      ethernet  connected     base0      
...


$ nmcli networking off

$ nmcli networking 
disabled

$ nmcli networking connectivity 
none

$ brctl show
bridge name     bridge id               STP enabled     interfaces
bridge0         8000.000000000000       no              

$ nmcli device 
DEVICE      TYPE      STATE      CONNECTION 
bridge0     bridge    connected  bridge0    
enp3s0      ethernet  unmanaged  --         
...