nng_sockaddr could just be a union

[gdamore]

The current definition of nng_sockaddr contains only one member, s_un.

This is kind of pointless, and makes for more verbose and uglier code.

It would be cleaner to define this as:

union nng_sockaddr {
uint16_t s_family; // ...
nng_sockaddr_in s_in; // ... and so on
};

typedef struct nng_sockaddr nng_sockaddr;