Fix a race where artifacts could print when exiting a program

This commit also consolidates the exit operations for consistency's
sake, and adds a kill() method to renderers for stopping them without
performing any final rendering.

nil_renderer.go

@@ -4,6 +4,7 @@ type nilRenderer struct{}
 
 func (n nilRenderer) start()              {}
 func (n nilRenderer) stop()               {}
+func (n nilRenderer) kill()               {}
 func (n nilRenderer) write(v string)      {}
 func (n nilRenderer) repaint()            {}
 func (n nilRenderer) altScreen() bool     { return false }