A Nic::Session client can install a signal handler that is used to
propagate changes of the link-state by calling 'link_state_sigh()'.
The actual link state is queried via 'link_state()'.
The nic-driver interface now provides a Driver_notification callback,
which is used to forward link-state changes from the driver to the
Nic::Session_component.
The following drivers now provide real link state: dde_ipxe, nic_bridge,
and usb_drv. Currently, OpenVPN, Linux nic_drv, and lan9118 do not
support link state and always report link up.
Fixes#1327
The port was succesfully tested a echo test and lighttpd. DHCP over
OpenVPN is not tested and probably will not work out of the box.
Therefore, the ip address etc. need to be specified manually.
For now, only ethernet bridging (using a TAP device) is supported.
Fixes#1235.
