Add UNI-T UT71x DMM parser.
Add support for the UNI-T UT71x series (aka Voltcraft VC-920/940/960).

Tested on the Voltcraft VC-920 and VC-940 (both UT-D02 and UT-D04
cables), but it should work for all devices in this series without
any changes.
Add config info for device type and limit config keys.
More robust searching for config keys.
atten-pps3xxx: Publish driver options.
cem-dt-885x: Publish driver options.
demo: Rearrange driver and device options.
scpi-pps: Publish driver options.
asix-sigma: Publish driver options.
Change SR_CONF_FILTER key to a boolean type.

This was an ill-defined string before, now it's simply something
you turn on or off on a channel.
hantek-dso: Fix driver/global/channel group config keys.

This was way behind and did not yet support channel groups.
Make sr_dev_inst_connid_get() available without libusb