ui/ncurses: Layout tweaks for boot order interface

ui/ncurses: Layout tweaks for boot order interface

This change implements a few minor tweaks for the bootorder layout
interface:

 - make the verical spacing of widgets more consistent (we have
   a couple of 3-line gaps)

 - use a consistent 2-space gap between buttons on the same line

 - clarify the title text for the subset widget instance, and
   move the select widget closer to the title

 - use consistent title-case for widget labels

 - left-align the select & button widgets on the subset screen, for
   consistency with other screens

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