expose explicit golang IPV4 and IPV6 handling (udp4 udp6)

[emiago]

There is need some work done, to allow caller to prefer ipv4 or ipv6 in some cases. As this is handled by std library, sipgo needs to expose and handle cases like tcp4, tcp6

DNS is now prefered with IPV4, we need to fix this.

Modern world is getting out of IPV4

[emiago]

server listeners have now support for passing udp4 udp6. More work needed for client

[emiago]

Client must determine this based on recipient or if based on local connection interface addr via Request Laddr. In first case it should be enough to deteremine raddr.IP