always clear pending command queue before posting shutdown commands

this ensures that, among other things, we won't get into this scenario:
- process is running
- interrupt is requested by user
- termination is requested by user
- before interrupt takes effect, the process terminates
=> shutdown is called
=> exit is also queued, as there is already a queued kill
=> nothing happens, as there will never be a stop response

Reviewed-by: hjk