sleep: if FANCY && DESKTOP, support fractional seconds, minutes,

sleep: if FANCY && DESKTOP, support fractional seconds, minutes,
 hours and so on. It's coreutils compat. bloatcheck is atrocious :(

function                                             old     new   delta
sleep_main                                            71     362    +291
bb_strtod                                              -     127    +127
make_device                                         1269    1294     +25
getoptscmd                                           708     713      +5
switch_root_main                                     402     401      -1
display_speed                                         90      85      -5
show_entry                                           295     289      -6
parse_expr                                           841     833      -8
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(add/remove: 1/0 grow/shrink: 3/4 up/down: 448/-20)           Total: 428 bytes