device: sr_dev_clear() now always works.

If the driver does not provide a dev_clear() function, the wrapper
now calls std_dev_clear() instead.
session: Implement dev_clear().
Remove std_dev_clear() wrappers from drivers.
session: Always read a whole number of samples.
brymen-bm86x: Initial driver skeleton.
brymen-bm86x: actual driver implementation
configure.ac: Move Brymen BM86X snippet to correct place.
brymen-bm86x: Remove std_dev_clear() wrapper.
Fixed session_driver so that it can run more than one session.
Fluke189 support

I can confirm that the 189 uses the same "QM" command as the 187:
<-- QM\r
--> QM,+03.225 mV AC
gmc-mh-1x-2x: Fixed 300 µA range scaling.
gmc-mh-1x-2x: Cosmetic fixes for Metrahit 25S.
visa: Fixed missing header warnings/uninitialized variable.
gmc-mh-1x-2x: Whitespace fixes (cosmetic).
brymen-bm86x: disable driver when libusb is not present