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).