MASTECH MS8250D support

libsigrok now supports the MASTECH MS8250D multimeter.

This is a USB-based, 6600 counts digital multimeter. It's connected to the PC via a Silicon Labs CP2102 USB to serial converter.

It uses an 18-byte custom protocol, the actual measurement values basically represent the 7-segment display(s), with some additional boolean flags in the remaining bytes.

The libsigrok driver for this DMM uses the common serial-dmm infrastructure, hooking up a newly written parser for the MS8250D protocol.

This driver has been contributed by Stefan Mandl, thanks a lot!