extend fractional duration support to "top -d N.N" and "timeout"

extend fractional duration support to "top -d N.N" and "timeout"

function                                             old     new   delta
parse_duration_str                                     -     168    +168
sleep_for_duration                                     -     157    +157
top_main                                             885     928     +43
timeout_main                                         269     312     +43
handle_input                                         571     614     +43
duration_suffixes                                      -      40     +40
sfx                                                   40       -     -40
sleep_main                                           364      79    -285
------------------------------------------------------------------------------
(add/remove: 4/1 grow/shrink: 3/1 up/down: 494/-325)          Total: 169 bytes

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