refactor nanoget_repaint() to split out the new function

refactor nanoget_repaint() to split out the new function
get_statusbar_page_start(), the statusbar prompt's equivalent of
get_page_start(); also make sure that the minimum allowed terminal size
in columns is 4, as the statusbar prompt code relies on this assumption
and will crash otherwise

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@3051 35c25a1d-7b9e-4130-9fde-d3aeb78583b8