uni-t-ut32x: migrate from USB transfers to serial-over-HID communication

uni-t-ut32x: migrate from USB transfers to serial-over-HID communication

Switch the UT32x driver from running specific USB transfers to generic
serial communication. Preset the bitrate and frame format, but allow for
user specified overrides as well. Default to the WCH CH9325 HID chip,
but allow for overrides or more specific selection so that users can
resolve ambiguities with multiple cables.

The switch from libusb to hidapi removes a limitation that specifically
was reported for the Mac platform. The serial-over-HID variant should
now work as well. See bug #555.

Drop the background transfers. Stick with a local acquisition stop
routine, because a STOP request needs to get sent to the device. Reduce
the receive buffer size such that either blocking or non-blocking calls
will work. The additional flexibility of the buffer handling and packet
processing does not harm.