Removed the Nic::Driver implementation. All nic servers now inherit from
Nic::Session_component. Packet stream signals are dispatched to
the 'handle_packet_stream' function within a session component. Thus, nic
servers now have direct access to the packet stream channels, making handling
more flexible.
Adjusted nic_loobpack, dde_ipxe, wifi, usb, lan9118, Linux nic, and OpenVPN to
the new interface.
Fixes#1602
