su: expand help; simplify passing of -c CMD to run_shell()

su: expand help; simplify passing of -c CMD to run_shell()

Also, added a comment about bug 9401 (TIOCSTI input injection).

function                                             old     new   delta
packed_usage                                       30909   30932     +23
su_main                                              470     487     +17
sulogin_main                                         260     258      -2
run_applet_and_exit                                  681     678      -3
run_shell                                            166     126     -40

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>