size_t fmt_humank(char *dest,unsigned long long source);
fmt_humank writes a human readable ASCII representation of source to dest and returns the number of bytes written. The result resembles the file size output of "ls -h"; 1024 becomes "1.0k", 1048576 becomes "1.0M" and so on for "G" and "T".
fmt_humank does not append \0.
If dest equals FMT_LEN (i.e. is zero), fmt_humank returns the number of bytes it would have written.
The output of fmt_humank can not exceed 11 (assuming unsigned long long has 64 bits).