Address multiple issues with strict rtsock message validation.
D28668 "normalisation" approach was based on the assumption that we always have at least "standard".
It turned out to be false - certain older applications like quagga or routed abuse sin_len field and set it to the offset to the first fully-zero bit in the mask. It is impossible to normalize such sockaddrs without reallocation.
With that in mind, change the approach to use a distinct memory buffer for the altered sockaddrs.
This allows supporting the older software while maintaining the guarantee on the "standard" sockaddrs.