uucore: Permit 'D' as a decimal suffix modifier in size parsing for GNU compatibility.

[aweinstock314]

truncate: Reject 'b' as a unit.

[sylvestre]

error: unresolved link to `parse_size_u64`
   --> src/uu/truncate/src/truncate.rs:301:40
    |
301 | /// A size string is as described in [`parse_size_u64`]. The first character
    |                                        ^^^^^^^^^^^^^^ no item named `parse_size_u64` in scope
    |

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!