Add special call to clear the status window after user input

[jonas]

Follow-up to aa84d85 to fix performance
regression noted in #649. Also cleans up open_prompt to not clear the
status window.

[rolandwalker]

This is nicer logic but doesn't address the performance regression in #649.

set_cursor_pos(0, 0) is the actual direct fix to the bug reported in #623. Readline is the problem. Ncurses fails to keep up with all the changes that may happen during readline world.

set_cursor_pos(0, 0) forces ncurses to sync some state, and it follows readline because that is the locus of the bug. getyx(newscr …) always returns incorrect values after Ctrl-C inside readline().

wclear() also forces ncurses to sync state, just expensively, because wclear() marks the entire screen dirty for redrawing. In this case we get a complete redraw on every keystroke.