Previously, "mono" assigned -1 to fg/bg to indicate "unset" values.

NCurses uses -1 to indicate "default" color (when supported), but
COLOR_DEFAULT was assigned the value -2 and swapped at the last
moment.  While it worked, I personally found this confusing.

To make the logic clearer, create a COLOR_UNSET constant with value
-2, and switch COLOR_DEFAULT to -1.  Then remove the last-minute
translation when allocating the color.

Change the "not set" initialization and testing to use COLOR_UNSET for
clarity.

Because map_getvaluebyname() returns -1 for a missing value,
create a new lookup function that returns the matching mapping_t or
NULL if not found.