Provide forward declaration for `struct sockaddr`

Otherwise, at least clang++, forward-declares it inside the namespace `::capnp` and doesn't match it with a real `struct sockaddr` from the header 'socket.h'.

Also, accept `const` pointer - there is no point not to do so.
Accept `struct sockaddr` as a const pointer.
Merge pull request #83 from abyss7/patch-1

Fix problem with the sockaddr struct.