Add pidfile and daemon support
Add pidfile and daemon support

- Add PID file handling
- Rename freq_t with mhz_t, which is less ambiguous
- Add exit codes EDAEMON, ECONFLICT, EPID
- Add show_settings() to display the current setup in verbose mode
- Add run_daemon(), which manages the pidfile, detaching from the terminal
  and the main while loop
- Terminate for SIGINT and SIGTERM

lonkamikaze/powerdxx,lonkamikaze/powerdxx