dhcpc: Use client IP address as source address for DHCP renew/rebind messages

dhcpc: Use client IP address as source address for DHCP renew/rebind messages

RFC2131 paragraph 4.1 states DHCP messages broadcast by a client prior to
that client obtaining its IP address must have the source IP address
field in the header set to 0.
Request messages transmitted in renewing and rebinding state need to use
the obtained IP address as source IP address in the header; this behavior
lines up with other implementations like ISC dhcp client.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>