ls: more correct handling of -c, -u

ls: more correct handling of -c, -u

function                                             old     new   delta
my_stat                                              302     318     +16
packed_usage                                       30977   30969      -8
display_single                                       928     910     -18
sortcmp                                              258     228     -30
ls_main                                              776     732     -44
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(add/remove: 0/0 grow/shrink: 1/4 up/down: 16/-100)           Total: -84 bytes

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