pbr: bugfix: fix IPv6 interface errors

* update license to AGPL-3.0-or-later
* rename pbr_get_gateway to pbr_get_gateway4 for better readability
* improve IPv6 "gateway" detection/display on start
* prevent IPv6 interface errors on start
* revert release format

Signed-off-by: Stan Grishin <stangri@melmac.ca>