implement DHCPDECLINE by ensuring the device gets a different IP

[stapelberg]

Turns out that Apple’s DHCP client sends DHCPDECLINE requests when it’s unhappy about a lease (e.g. because it thinks the IP address is still in use by some other device).

We should implement that and ensure giving out a different address for subsequent requests. This will result in a better failure mode.

[stapelberg]

We’re already using rand.Intn to avoid handing out the same IP address:

router7/internal/dhcp4d/dhcp4d.go

Line 152 in 53c4950

i := rand.Intn(h.leaseRange)

…so I’m thinking that we would also need to delete all previous leases for this MAC address from the database to avoid hitting this code path:

router7/internal/dhcp4d/dhcp4d.go

Line 286 in 53c4950

// offer previous lease for this HardwareAddr, if any