networkd: DHCP override hostname

This patch enhances the DHCP client to  send the hostname
reference
 http://lists.freedesktop.org/archives/systemd-devel/2014-July/021550.html

Tested with
Example conf:

[Match]
Name=eth1

[Network]
DHCP=v4

[DHCP]
SendHostname=true
Hostname=test