Fix IPAddress packed layout and iovector flex array

[Coldwings]

In issue #350

Shows new version GCC performs more check for old GNU extensions, and treats union as non-POD member in struct layout.

Here some fixes for them, tested worked in GCC 13.2 and fits the requirements of C++ standard:

1. Use flex array instead of deprecated zero-sized array which causes warning.
2. Prevent inherent to a struct with flex-array.
3. Add attribute for union in IPAddr, make IPAddr struct able to be packed

[lihuiba]

[0] is a common practice, and it's more read-able than iovs_ptr(). We can simple ignore the warning with #pragma.

[Coldwings]

It is a common solution in GNU C extension.
C99 standard add new define of flex array, struct iovec iovs[] to present a flex array.
though, C++ does not allow inheritance by a class with tailed flex array, that is why here replaced by iovs_ptr()

Seems there is not a better solution than iovs[0]