libbb: shrink monotonic_XXX functions, introduce monotonic_ns

libbb: shrink monotonic_XXX functions, introduce monotonic_ns
(unused for now)

function                                             old     new   delta
get_mono                                               -      31     +31
sv_main                                             1228    1234      +6
expand                                              1693    1697      +4
get_address                                          178     181      +3
utoa_to_buf                                          108     110      +2
builtin_exit                                          46      48      +2
qrealloc                                              36      33      -3
qgravechar                                           109     106      -3
ash_main                                            1383    1380      -3
grep_file                                            850     846      -4
popstring                                            140     134      -6
monotonic_us                                          85      60     -25
monotonic_sec                                         41      16     -25
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/7 up/down: 48/-69)            Total: -21 bytes