asix-omega-rtm-cli: Implement RTM for ASIX OMEGA via external CLI process

asix-omega-rtm-cli: Implement RTM for ASIX OMEGA via external CLI process

The ASIX OMEGA vendor software provides a commandline application which
puts the device in "real time mode". The process' stdout provides a
continuous RLE compressed stream of samples for the 16 input signals,
recorded at 200MHz. The sigrok driver starts and terminates this process
for the duration of the acquisition.

This simple approach makes the OMEGA device available in a basic mode of
operation. The samplerate is fixed, hardware triggers are not available.
The binary data format is used to reduce the amount of inter process
communication. The vendor's Windows software also executes in Linux
under wine(1). All device detection, USB communication via FTDI FIFO
mode, firmware download, etc are transparently dealt with. This mode of
operation is an officially supported and documented feature.

The sigrok driver accepts the OMEGARTMCLI environment variable as the
specification of the vendor application's location, or falls back to the
omegartmcli executable name which should be in PATH. The conn= spec in
the sn= format can select one out of multiple connected devices.

The driver was tested on Linux with --samples and --time as well as with
Pulseview and manual acquisition stop while a huge limit was configured
that would not take effect.