(In progress) Define port_t type for endpoint's port number

[tiendq]

Just inspired by size_t, would it be a little more readable to use port_t instead of unsigned short? If yes I'll check and update everywhere.

And it can be used in consumer code too, for example:

class ServerBase {
public:
  ServerBase(boost::asio::port_t port = 8000) {}
};

[viccpp]

POSIX calls this type in_port_t. http://pubs.opengroup.org/onlinepubs/000095399/basedefs/netinet/in.h.html