Whoops, one of the intermediate changes I'd made while working on these 
patches broke a bunch of the regression tests, so here's the fixed 
version. I also renamed a few of the new variables to be clearer that 
they're for internal use only.

Since it sounds like all the other concerns have been addressed, I'll 
probably merge this in the next day or so. (This is the last non-bugfix 
change I'd like to land for Eshell in Emacs 30.)