Various fixes.

Various fixes.

Get rid of zero-length arrays; not all compilers support them.

Get rid of __attribute__((packed)); not all compilers support it, so we
can't depend on it.

Use explicit constants for sizes, just in case some compilers pad
structures.

Add some length checks.

Make some ND_TTEST2 calls ND_TCHECK2 calls instead, so they report
truncation if they fail.

Constify some pointers.