stat: improve GNU compatibility (#6933)

* stat: fix the quotes when dealing with %N

and other formats
should fix
tests/stat/stat-fmt.sh

* stats: use an enum instead of a string

* stats: split the functions into smaller functions

* stat: handle byte as a format for better display

* stat: handle error better. should make tests/stat/stat-printf.pl pass

* stat: Some escape sequences are non-standard

* Fix tests

* Take comments into account

Author: sylvestre