Fix the unknown flags handling when receiving such message. We should
zero out all of the Z bits in the OPT record before returning.

Current behavior:

    dig +norec +noad +ednsflags=0x80 soa miek.nl @deb.atoom.net
    ...
    ; EDNS: version: 0, flags:; MBZ: 0080 , udp: 4096

New:

    dig +norec +noad +ednsflags=0x80 soa miek.nl @localhost -p 2053
    ...
    ; EDNS: version: 0, flags:; udp: 4096

Take care no to overwrite the Do bit.

We still accept *all* EDNS option; I do not consider that a bug in
itself.

Fixes #306