Change struct bgpd_addr VPN encoding. Instead of including two almost

Change struct bgpd_addr VPN encoding. Instead of including two almost
equal versions put the RD and lable stack right into struct bgpd_addr.
For non-VPN addresses these extra fields are ignored. Since VPN and non-VPN
addresses encode the prefix in the same way now some code can be simplified.
In most cases a fallthrough or reuse of encoding functions is now possible.
It should also reduce the size of struct bgpd_addr a bit.
OK denis@