ui/ncurses: Use a fixed-sized for small, known-length fields

Fields without O_STATIC can "scroll" horizontally, and we may miss a
left-hand section of text from the current display. This can mean that
the user can't leave a validated field with no indication why, if the
off-to-the-left data is not a valid entry.

This change adds a widgetset function to mark a field as fixed-size, so
we don't have this scrolling behaviour. This means that the entire field
contents will always be visible, and any validation errors can be seen.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>