Avoid `snprintf()` in Preparing Human-Readable Sizes; Improve Formatting

Avoid `snprintf()` in Preparing Human-Readable Sizes; Improve Formatting

This produces the following formatting:

   Size    | `zstd` | `ls -lh`
---------- | ------ | --------
1          | 1      | 1
12         | 12     | 12
123        | 123    | 123
1234       | 1.21K  | 1.3K
12345      | 12.1K  | 13K
123456     | 121K   | 121K
1234567    | 1.18M  | 1.2M
12345678   | 11.8M  | 12M
123456789  | 118M   | 118M
1234567890 | 1.15G  | 1.2G
999        | 999    | 999
1000       | 1000   | 1000
1001       | 1001   | 1001
1023       | 1023   | 1023
1024       | 1.000K | 1.0K
1025       | 1.00K  | 1.1K
999999     | 977K   | 977K
1000000    | 977K   | 977K
1000001    | 977K   | 977K
1023999    | 1000K  | 1000K
1024000    | 1000K  | 1000K
1024001    | 1000K  | 1001K
1048575    | 1024K  | 1.0M
1048576    | 1.000M | 1.0M
1048577    | 1.00M  | 1.1M

This was produced with the following invocation:

```
for N in 1 12 123 1234 12345 123456 1234567 12345678 123456789 1234567890 999 1000 1001 1023 1024 1025 999999 1000000 1000001 1023999 1024000 1024001 1048575 1048576 1048577; do
  head -c $N /dev/urandom > r$N
done
./zstd -i1 -b1 -S r1 r12 r123 r1234 r12345 r123456 r1234567 r12345678 r123456789 r1234567890 r999 r1000 r1001 r1023 r1024 r1025 r999999 r1000000 r1000001 r1023999 r1024000 r1024001 r1048575 r1048576 r1048577
```