Improvement: rewrite stdinargs

[balupton]

Currently, both of these:

echo-lines | echo-verbose
echo-lines '' | echo-verbose

Will output nothing. This is due to the bash v3 check for ARGS existence that we have, that checks if ARGS is an empty string.

The other issue is having to do env TIMEOUT=... echo-* rather than echo-* --timeout=....

The other one is things like had_stdin and other shared vars. That could be done instead via args to a process_stdinargs function, and for sharing vars back to the caller via the technique we use eval_capture.