Whilst experimenting with something else, I discovered that "zargs -P"
would often abandon its backgrounded jobs when receiving
terminal-driven signals.  This led me down several rat-holes trying to
figure out what was going on.  In fact I'm still not sure what all the
reasons were or how many of them were related to "setopt nomonitor"
(which is removed in the attached patch).

I discovered for example that even when INT and QUIT were explicitly
trapped, a shell function passed as the command for zargs to run would
execute with those signals either reset to their defaults or simply
ignored, and I haven't been able to work out the circumstances when
each of those cases occurs.  E.g., with

f() { sleep $1; print $1 }
zargs -n 1 -P 3 -- 2 3 4 5 6 -- f

Interrupting zargs with ^C would leave all the background "f" running,
waiting for "sleep".  On the other hand, with

zargs -n 1 -P 3 -- 2 3 4 5 6 -- /bin/sleep

the ^C terminated the sleeps as expected but attempting to stop zargs
with ^Z would cause zargs to return, leaving orphan stopped sleep
processes behind.  That part was due to "nomonitor".

Here then is a patch which:
-- as discussed in the "Removing subshell from zargs" thread, removes
the preliminary "wait" for all the process
-- as noted above, removes "nomonitor" (because it was only needed for
that "wait")
-- explicitly adds traps to exit for tty-generated signals plus TERM,
except that it ...
-- captures the global signal trap context and restores it in the
background jobs
-- wraps the whole thing in an "always" block to clean up local helper functions
-- updates the comments to note another buglet and drop support for zsh 4.x.

As further speculated in the other thread, this doesn't yet remove the
subshell around the whole mess.

I'd love to hear explanations of why the signal handling behaves as it
does.  Every time I thought I had the logic mapped onto subshell entry
conditions or whatever, it'd behave a different way the next time I
tried, until I ended up with the rather brute-force approach in the
patch.