knock

First refactoring: Adding tests for list.c making sure that elements are freed properly. Also fixed mem issues in knockd.c.

Added fix for VLAN Ethernet as proposed here: https://github.com/jvinet/knock/issues/89. Fixed log output.

Fix issue in 'any' interface handling

Add support for using 'any' for the configured interface to listen on all network interfaces of the device

Address review comments of pull request jvinet/knockd#84