wifi: don't touch by default current powersave setting

wifi: don't touch by default current powersave setting

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