dhcp: Add DHCP discover sending

dhcp: Add DHCP discover sending

On starting the client, use the supplied interface mac address and create
a transaction id. Puzzle together an IP/UDP/DHCP Discover message, compute
checksums and send it out as a raw packet.

Create an additional function that constructs default options common to
all DHCP messages.

Set the DHCP Client ID option as noticed by Grant Erickson in ConnMan
commit b18d9798b3a0ae46ed87d6d2be8d5a474bf3ab1e:

   "Some Internet gateways and Wi-Fi access points are unhappy when the
    DHCPv4 client-id option (61) is missing and will refuse to issue a
    DHCP lease."