ui/ncurses: Add nc-subset selection screen

The nc-subset screen is intended to be used as a sub-screen from the
current screen (eg. nc-config) which passes a pointer to a
nc_widget_subset struct. The nc-subset screen allows the user to select
an option from a list of 'inactive' options, before returning control
back to the current screen.

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