- tested that it hangups when user's program exits: DONE
- tested that it closes the pty when user hangups: DONE
- tested binary SEND and CPU usage:
     file ok, CPU usage is quite high.  DONE
- tested binary RECEIVE and CPU usage:
     file ok, CPU usage is far lower than before. DONE
     (so maybe a problem in the selection loop
      when a lot of data goes TO the modem through ptylogin, ie write).
- tested lastlog/utmp:
     - in exit case:  *BUG*: not updated
     - in hangup case: not tested
- tested correct mode/owner on tty: yes.  DONE