libbb: introduce and use monotonic_ms

libbb: introduce and use monotonic_ms

function                                             old     new   delta
monotonic_ms                                           -      60     +60
process_stdin                                        433     443     +10
display_speed                                         85      90      +5
nmeter_main                                          672     674      +2
builtin_type                                         114     116      +2
bb__parsespent                                       117     119      +2
ifplugd_main                                        1110    1109      -1
acpid_main                                           441     440      -1
chat_main                                           1361    1359      -2
doCommands                                          2458    2449      -9
arpping                                              466     450     -16
run_command                                          268     234     -34
readcmd                                             1072    1034     -38
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/7 up/down: 81/-101)           Total: -20 bytes

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