libbb/human_readable.c: shrink; and reduce bss usage

libbb/human_readable.c: shrink; and reduce bss usage

also, move smart_ulltoaN there and comment usage locations

function                                             old     new   delta
static.unit_chars                                      7       9      +2
utoa_to_buf                                          110     108      -2
make_human_readable_str                              262     258      -4
fallbackSort                                        1723    1719      -4
static.fmt                                            97      92      -5
static.fmt_tenths                                     10       -     -10
static.str                                            21       4     -17
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/5 up/down: 2/-42)             Total: -40 bytes
   text    data     bss     dec     hex filename
 820981     453    6932  828366   ca3ce busybox_old
 820968     453    6916  828337   ca3b1 busybox_unstripped

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