Set source IP address in PktFilterInet::send

Perfdhcp can be used with option -l to specify the local address to be
used.

However, when this option is used on a secondary address of an
interface, this doesn't work correctly.

The packets are sent with:
- giaddr = adresse specified (correct)
- source ip address = the interface primary IP address

Here is a patch that correctly sets the source IP address before sending
the packet.
This allows to launch multiple perfdhcp processes at the same time,
sharing the same interface but each using a distinct IP address.