Currently, it's not possible to call "(unload-feature 'eshell)" after 
using Eshell. You'll just get an error that some of Eshell's extension 
modules depend on eshell.elc. This is a relatively minor issue, but I'm 
hoping to make some improvements to how extension modules get loaded, so 
this is a prelude to that.

In addition to making it *possible* to unload Eshell, I also fixed an 
issue where the unload hooks weren't named correctly. They were of the 
form 'eshell-hist-unload-hook', but the file's name is em-hist.el. I 
moved these to 'em-hist-unload-function' and similar ('-function' 
because the '-hook' version is obsolete, as I understand it).