This actually makes very little difference at the moment, but will make
things more confortable later on, when the logic of enabling/disabling
coloring will involve terminal-colors.d(5).

Instead of deciding whether to use colors lazily with use_colors(), it's
done very early on nmcli initialization and a boolean use_colors field
is stored in the NmcConfig instance instead of the raw tristate option
of NmcColorOption type (which is now confined to nmcli.c).

Wherever the NmcColorOption was used previously, the whole NmcConfig
instance is passed around. That might seem pointless (since only the
use_colors boolean is actually used at the moment), but will be utilized
to pass around the actual color palette in future.