setup: Allow backspace or del to clear controls.

Backspace or delete clears other input boxes; make it do the same for
keyboard, mouse and joystick inputs.
textscreen: Fix jump when clicking on scrollbars.

Clicking on a scrollbar should scroll the cursor to that location, but
the logic for it was buggy because of loss of precision when doing an
integer divide. When dividing by bar_max, seek up to the nearest value
so that the cursor always arrives where we clicked.

Thanks to Alexandre-Xavier for reporting this bug. This fixes #359.