Set src IP on more situations.

If the admin set "ipaddr = *", and no "src_ipaddr = ...", then
we used to send packets from 0.0.0.0.  That's wrong.

We now look for server identifier or server IP address.
If one of those is found, we use that as the source IP
Document src_ipaddr in more detail
Port DHCP fixes from v3.0.x

for giaddr address and port.