Fix some narrowing warnings on LP64/LLP64 platforms.

Fix some narrowing warnings on LP64/LLP64 platforms.

Add a ND_BYTES_AVAILABLE_AFTER() macro to find the number of bytes
available in the captured data, starting at the byte pointed to by the
argument.  It returns a u_int rather than a ptrdiff_t, so it'll be
32 bits on LP64 and LLP64 platforms as well as on ILP32 platforms.  Use
that macro.

Make size-of-buffer arguments size_t.

Cast some size_t and ptrdiff_t values to u_int or int.