Clean up signed vs. unsigned.

Clean up signed vs. unsigned.

Use unsigned for lengths.  Add additional length checks, as we can no
longer cheat and rely on "underflow turns the length negative, so just
check for > 0".

Show most NBT and SMB numerical fields as unsigned, as that's what they
are.  Have "d" and "D" treat the value as signed; add "u" and "U" for
unsigned values.