ls: make --color more compatible with coreutils

ls: make --color more compatible with coreutils

function                                             old     new   delta
static.color_str                                       -      34     +34
static.ls_longopts                                     -       9      +9
ls_color_opt                                           9       -      -9
ls_main                                              865     843     -22
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/1 up/down: 43/-31)             Total: 12 bytes
   text    data     bss     dec     hex filename
 820145     450    7692  828287   ca37f busybox_old
 820144     450    7692  828286   ca37e busybox_unstripped

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