Use a symbolic name for the "user specified input threshold" item's
position in the set of choices. Stick with array size for the complete
list of items instead of hiding the size behind an unrelated name. This
improves readability of the config get/set/list routines. Trim a text
line length while we are here.
The single variable for several purposes (user specified value, and
fixed value derived from currently selected choice out of a premade
set) still looks suspicious, and is not addressed in this commit.