Tests: Fix test_dbss, it should not be using a reserved channel for its database.
Tests: Add example unit test for Config variable constraints.
Bugfix: Main should only attempt to change directory if there is a directory to change to.
Config: Implement framework for ConfigVariable constraints.
Config: Add reusable constraints to a "constraints.h" header.
Config: Statisfy the test_config_dbss unittest by adding constraints to the DBSS config variables.