Draft: Make UT61D and VC-820 "first-class" drivers.

Draft: Make UT61D and VC-820 "first-class" drivers.

The 'uni-t-dmm' driver/directory will not appear as a "driver" to
frontends anymore, it's just an internal thing.

The frontends will see a uni-t-ut61d and voltcraft-vc820 driver now,
with the correct names and parsers etc. attached to them.

This is not fully finished yet, but it's a start (and works mostly):

  $ sigrok-cli -D
  The following devices were found:
  UNI-T UT61D with 1 probe: P1
  Voltcraft VC-820 with 1 probe: P1

  $ sigrok-cli --driver voltcraft-vc820 -D
  The following devices were found:
  Voltcraft VC-820 with 1 probe: P1

  $ sigrok-cli --driver uni-t-ut61d -D
  The following devices were found:
  UNI-T UT61D with 1 probe: P1

  # Now attaching a UNI-T UT61D device via USB.

  $ sigrok-cli --driver uni-t-ut61d --samples 3 -O analog
  P1: -0.017800 V DC
  P1: -0.017600 V DC
  P1: -0.017700 V DC

  # Now attaching a Voltcraft VC-820 device via USB instead.

  $ sigrok-cli --driver voltcraft-vc820 --samples 3 -O analog
  P1: -0.319200 V DC
  P1: -0.319300 V DC
  P1: -0.319300 V DC