interactive: Implement single key mode.

interactive: Implement single key mode.

This changes the i9e subsystem to operate in single key mode while
a command is running as determined by the existence of a buffer tree
node which is attached to the stdout_btrn of i9e.

In single key mode a different readline key map is activated and
single keystrokes are passed to the application via the new key
handler function pointer of struct i9e_client_info.