dos2unix: tiny shrink

dos2unix: tiny shrink
login,su: fix setup_environment() so that it works as intended
          (parameter names were a bit misleading)
fdisk: shrink
help text: shrink

function                                             old     new   delta
login_main                                          1658    1701     +43
setup_environment                                    206     203      -3
dos_compatible_flag                                    4       1      -3
dos2unix_main                                        383     375      -8
get_boot                                            1724    1702     -22
fdisk_main                                          2949    2889     -60
packed_usage                                       24250   23948    -302
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/6 up/down: 43/-398)          Total: -355 bytes
   text    data     bss     dec     hex filename
 798768     661    7428  806857   c4fc9 busybox_old
 798327     658    7428  806413   c4e0d busybox_unstripped