Change subprocess exit code detection to return 128 + signal

[kfeinauer]

We recently made a change in the upstream RSP code to report process exit codes as 128+signal if the process exited due to a signal. This is a bash paradigm that is fairly widely recognized.

Update the SDK subprocess exit code detection mechanism similarly to what was done for https://github.com/rstudio/rstudio-pro/issues/2045.