ui/ncurses: Simplify starting shell

Instead of calling sh twice to echo the 'exiting' message just call sh
by itself and leave any welcome message to the shell's interactive
config.
Also drop the explicit nc_scr_unpost() in cui_run_cmd() since clear()
will blank out the screen anyway.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>