* new config option for external address to advertise
* if blank, defaults to best guess from listener
* if laddr ip address is also blank, default to IPv4
* config: rename skip_upnp to upnp
Change default option to enable upnp.
Closes#1806
* doc updates
- fix comment and set UPNP to false in TestP2PConfig
- add UPNP to config template
- update changelog