networkd: send hostname to dhcp server

Send hostname (option 12) in DISCOVER and REQUEST messages so the
DHCP server could use it to register with dynamic DNS and such.

To opt-out of this behaviour set SendHostname to false in [DHCP]
section of .network file

[tomegun: rebased, made sure a failing set_hostname is a noop and moved
          config from DHCPv4 to DHCP]
networkd: netdev - add dummy support