Some drivers (or things outside NM like 'powertop') may turn powersave
on, so don't touch it unless explicitly configured by user.

To achieve this, add new 'default' and 'ignore' options; the former
can be used to fall back to a globally configured setting, while the
latter tells NM not to touch the current setting.

When 'default' is specified, a missing global default configuration is
equivalent to 'ignore'.

It is possible to enable Wi-Fi power saving for all connections by
dropping a file in /etc/NetworkManager/conf.d with the following
content:

 [connection]
 wifi.powersave=3

https://bugzilla.gnome.org/show_bug.cgi?id=760125