- improve command line arguments fetching? (inspired from wc?)
- factorize code; either:
  * need to improve configure;
  * include a .c files;
  * write a library;
  * use libSystem;
- decide how to include the utils only defined in XSI, at the moment:
  * du
  * link
  * nice
  * renice
  * strings
  * time
  * unlink
  and for those with special features, at the moment:
  * ls
  => either:
    . include always;
    . build conditionally;
    . improve configure to allow conditional targets;

id:
- write more readable code
- check if it's complete

kill:
- get signal number from its name

mkdir/rmdir:
- check recursive algorithms