base

Connect the DHCP UDP socket to the server.

This makes it so that the socket cannot receive datagrams from
anybody except the DHCP server. This does not improve security,
because we never read from the UDP socket anyway, but it does
make ListeningPortsTest pass.

Bug: 23906864
Bug: 23933386
Change-Id: I82fe9d6c6c520536ffd6422bcc60fab664999e6f