all:
- factorize code (either need to improve configure or include a .c file)
- improve command line arguments fetching? (inspired from wc?)
- factorize code; either:
  * include .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;

id:
- check if it's complete

kill:
- get signal number from its name

ls:
- race condition between looking if is a directory and opendir()? (check cksum too)

rm:
- check for symlinks in recursive removal?

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

kill:
- get signal number from its name

mkdir/rmdir:
- check recursive algorithms

tail:
- implement