Hameg HMO scope support and SCPI backend code

The list of contributed hardware drivers for libsigrok is getting longer yet again. This time, support for the Hameg HMO series oscilloscopes (tested on Hameg HMO1524) has been added by poljar (Damir Jelić), thanks a lot! It has recently also been successfully tested on a Hameg HMO2024 at 30C3 in Hamburg.

This driver should support the Hameg HMO compact series device (70MHz - 200MHz) for now, other devices can be added later though. It also makes use of the recently added probe groups feature to allow setting coupling, trigger slope, timebase and so on independently for each scope channel. Currently the driver supports the serial port connectivity of the scope.

Since SCPI commands are used for controlling and querying the scope, poljar also added an initial set of common SCPI related functions to libsigrok, which other drivers can also make use of later. Those functions initially assumed a serial port transport, but have been made more generic in the mean time by Martin Ling (thanks!), now supporting SCPI over USBTMC and SCPI over TCP too.