settables: calculate the maximum width of the "Value" column

[gjvc]

When printing the table of settables, the "Value" column is limited to 30 characters, and the "Description" column to 60.

Calculate the maximum value of the "Value" column and use that value as the width, using the length of the title "Value" as a lower bound.

For the "Description" column, keep the current behaviour, though it really doesn't require a limit at the moment, being the rightmost column.