DHCP: Handle option 108 correctly when receiving 0.0.0.0 OFFER (#342)

DHCP: Handle option 108 correctly when receiving 0.0.0.0 OFFER (#342)

* DHCP: Handle option 108 correctly when receiving 0.0.0.0 OFFER

According to RFC8925 section 3.3.1, when the server supports both option
108 (IPv6-Only Preferred) and option 116 (Auto-Configure), and the
client only sends IPv6-Only Preferred option, then the server SHOULD
return 0.0.0.0 as the offered address, and not setting the
Auto-Configure option.

However, in our current client code, the IPv6-Only Preferred option in a
0.0.0.0 OFFER is only handled correctly when the Auto-Configure option
is present. This patch fixes this issue.

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Co-authored-by: Roy Marples <roy@marples.name>