[ref] listdir, filter, empty dir

[ref] listdir, filter, empty dir

- listdir() get arg int to highlight first entry
- check listdir() return value, print strerror < 0
- set_panes() add enum instead of 0 1
- space after prompt msg is 1
- simplify switch_pane()
- set_panes() start() return void
- fix empty dir: disable any movement except going back

* config.h
- add new file default permissions