Add support for IPv4 link-local addresses

This branch adds support for IPv4 link-local addresses. Previously the LAN plugin would allow connection attempts between unrecognised address prefixes because I wasn't sure which prefixes might be used for link-local addresses. Now only recognised prefixes are allowed. This shouldn't make any practical difference on Android, but it avoids a potential metadata leak on other platforms, if the device is on a LAN that provides a public IP address.