run_external_process() contained pipe file descriptors leaks (e.g. one
pipe end was never closed). Also the stdout might have been captured
incomplete, since only a single read() was performed on the pipe.
Furthermore should a child process try to write a larger amount of data
onto the pipe then it will become stuck, because the parent process
isn't consuming the data. Thus the timeout would trigger in these cases
although the child process does nothing wrong.
This commit changes the implementation to follow a select() based
approach that continually reads from the pipe, but discards data that
doesn't fit in the provided buffer.
