guest: Use separate routing table for guest network routes
Description
I case that router acts as VPN client, the guest network does not have access to the Internet. The Internet connectivity is re-enabled when the VPN client disconnects.
Proposed solution
From comment turris/os/packages!876 (comment 242659)
The VPN client and Guest network share the same (main) routing table. That means, when the VPN clients rewrites the default route to go through the VPN, the Guest network will lose the access to the Internet (as the default is going via VPN).
I suggest to move the VPN client to the separate routing table - that means the VPN client installs the default route to go through the VPN, but as it is a different routing table, it won't affect the main routing table; thus the Guest network will keep the access to the Internet but no access to the VPN)
Apart from using separate routing table, perhaps additional changes to firewall would be necessary.