Improve check for rig link during startup

- Report any error during startup of hamlib rig link
- In case of errors ask if user want to continue without rig control.
  Otherwise exit tlf.
- Drop unneeded sleep() calls
Improve handling of temporary drop of rig link

- If get_frequency fails (maybe because rig is switched off) report
  frequency as 0.0. Do not change band settings from tranceiver.
- Show and log the actual frequency as 0 - meaning no rig control
  available at the moment.
- writecabrillo() will insert start-of-band frequency in log calculated
  from band information