dhcp: don't stop receiving packets when the link goes down

dhcp: don't stop receiving packets when the link goes down

When the link goes down, DHCP client_receive_message*() functions return an
error and the related I/O source is removed from the main loop. With the
current implementation of systemd-networkd this doesn't matter because the DHCP
client is always stopped on carrier down and restarted on carrier up. However
it seems wrong to have the DHCP client crippled (because no packet can be
received anymore) once the link goes temporarily down.

Change the receive functions to ignore a ENETDOWN event so that the client will
be able to receive packets again after the link comes back.