interactive: Introduce i9e_print_status_bar().

interactive: Introduce i9e_print_status_bar().

When operating in single key mode, the users of the i9e API
may write a single line, called status bar, to stderr.

This patch adds a new public function to the i9e subsystem for
this purpose. It honors the width of the terminal and shortens
the given status bar string to fit in one line if necessary.