Make it possible to start the server without all the configured interfaces ready (GH#91)

<Originally reported on Github as issue #91 (closed) by karaluh on July 6, 2018>

Dibbler has an "inactive-mode" option, which works like this, according to the official docs:

During normal startup, client tries to bind all interfaces defined in a configuration file. If such attempt fails, client reports an error and gives up. Usually that is best action. However, in some cases it is possible that interface is not ready yet, e.g. WLAN interface did not complete association. Dibbler attempt to detect link-local addresses, bind any sockets or initiate any kind of communication will fail. To work around this disadvantage, a new mode has been introduced in the 0.6.0RC4 version. It is possible to modify client behavior, so it will accept downed and not running interfaces. To do so, inactive-mode keyword must be added to client.conf file. In this mode, client will accept inactive interfaces, will add them to inactive list and will periodically monitor its state. When the interface finally goes on-line, client will try to configure it." My use case is PD over PPP interfaces which come and go as they please during the server process lifetime and mostly aren't there when the server starts.