Prepare for a change in the GGU protocol

Issue #602, which is scheduled for Gatekeeper v1.3, requires a change of the GGU protocol. This change should be done now, so the protocol change already happened when issue #602 is worked out.

The proposal here consists of Gatekeeper servers accepting versions 1 and 2 of the protocol, but having an identical protocol. Whereas Grantor servers would use protocol version 2. The version 2 protocol would not use the decision types `GGU_DEC_IPV4_DECLINED`, `GGU_DEC_IPV6_DECLINED`, `GGU_DEC_IPV6_GRANTED`, and `GGU_DEC_IPV6_GRANTED`. Grantor servers can avoid these decision types by redirecting all to BPF programs. Thus, protocol version 1 can be dropped when issue #602 is worked out.