Implement basic configuration framework API exposed to tempesta modules.

This commit introduces three major entities:
1. TfwCfgMod - allows tempesta modules to subscribe to configuration
   changes and start/stop commands passed via sysctl.
2. TfwCfgSpec - allows modules to provide declarative specification
   for modules, set default values and callbacks for configuration
   entities.
3. tfw_cfg_userspace_if.c - The user-space interface that creates the
   sysctl tables, listens for start/stop commands, reads the config
   file and pushes it to registered modules.

Note that changes are still raw, there are many major TODOs in there.

Related to: #18 - Tempesta FW configuration framework