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
2 files changed