ui/ncurses: Improve scrolling behaviour

The nc-config screen now includes several select options with a FIELD
height greater than one, but config_screen_widget_focus() will only
scroll down enough to see the first line of text.
Update config_screen_widget_focus() to try and make as much of the
focussed widget visible as possible.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>