ui/ncurses: Add status log UI

Currently, status messages from the server are displayed in a single
line at the bottom of the main menu UI, and are lost once a new status
is reported.

This change adds a facility for the UI to collect and display the status
messages from the server, in a dedicated UI screen. This allows a user
to look back through the discovery & boot process.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>