dvbsrc: improve timeout handing at locking loop

New approach attempts to be more accurate by measuring
the elapsed time by iteration. Also:

* Use a 10 seconds default timeout and a half a second
  polling step. New values should better match the tuning
  process on real-life scenarios.
* Correct elapsed_time computation.
* Add _retry_ioctl() to avoid bailing out on temporary
  ioctl EINTR failures (no need to check for EAGAIN cause
  we are opening the frontend on blocking mode)
* Small corrections to fail condition handling
dvbsrc: drop redundant assignment