libbb: make history saving/loading concurrent-safe

* all history writers always append (not overwrite) history files
* they reload history if they detect that file length has changed since last
write
* they trim history file only when it grows 4 times longer than MAXLINES
* they do this atomically by creating new file and renaming it to old

Unfortunately, this comes at a price:

function                                             old     new   delta
load_history                                           -     346    +346
read_line_input                                     3155    3358    +203
new_line_input_t                                      17      31     +14
...irrelevant small jitter...
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(add/remove: 1/0 grow/shrink: 5/5 up/down: 573/-13)           Total: 560 bytes

0 files changed, 0 insertions, 0 deletions