many services depend on other services that bring up network interfaces.
Examples are ipv6 tunneling clients or VPNs.
As there are multiple choices for these network-interface-providing services,
it's not nice to hardcore these deps in every service.
This change sets up a generic config option for this purpose.
providers (gw6c/gogoclient/openvpn) can plug into this to signal they bring up
an important interface.
Daemons that need these interfaces, can then depend on the 'all-interfaces' event,
instead of the individual services.
By default, the event fires when network-interfaces completes.
svn path=/nixos/trunk/; revision=32764
77240b46f1