Since commit 87a3df2e, the unmanaged
flag NM_UNMANAGED_USER_SETTINGS could be overwritten via an explict
user decision (NM_UNMANAGED_USER_EXPLICIT).

It makes sense to allow user configuration from file to be changable
by an explict user action via D-Bus at runtime.

However, it also changes behavior for devices that are currently explicitly
managed. Previously, a reload of the NM_UNMANAGED_USER_SETTINGS would
immediately unmanaged the device:

  - for keyfile: send SIGHUP to reload NetworkManager.conf
  - for ifcfg-rh: `nmcli connection [re]load`

So this change in behavior could negatively affect users who rely
on being able to configure "NM_CONTROLLED=no" and expect to unmanaged
the device immediately. Thus revert the change.

Note that NM_UNMANAGED_USER_SETTINGS is anyway ugly and should be
deprecated:
  - for keyfile, why having the option "keyfile.unmanaged-devices"
    instead of a generic options?
  - for ifcfg-rh, why put per-device configuration in a per-connection
    file?
The preferred way is to configure NM_UNMANAGED_USER_UDEV via
"ENV{NM_UNMANAGED}". Maybe we should also add a new configuration
scheme via NetworkManager.conf.

https://bugzilla.gnome.org/show_bug.cgi?id=762331