zargs parallelization support blows up with |&

[Dima Kogan <dima@secretsauce.net>]

Hi. I can use this to invoke a perl script 10 times, 2 processes at a
time:

 zargs -P2 -i -l1 `seq 10` -- perl -e 'sleep 20' {}

This works ok. However, when I do the same thing, but filter stdout and
stderr for some output, then ALL the processes are invoked at the same
time, not just 2 at a time like I asked:

 zargs -P2 -i -l1 `seq 10` -- perl -e 'sleep 20' {} |& grep x

I don't know if this is a bug or an expected consequence of some
internal implementation details, but this is a really nasty failure
mode.

Thanks for looking.
dima