Proposal to not `quit` when we are working inside Slime

Hello there,

I tried defmain, and I tried to call the main function from Emacs & Slime. It should be possible, since what defmain does is creating a `defun` called `main` which accepts a `&rest argv`. My goal was to call the main function from a run.lisp script. That way I could avoid writing another function that would be called by main and that would accept the same key arguments from main.

So I evaluated (C-x C-e) or compiled a line like `(mypackage::main :debug t)` to see what it does, but it closes the Lisp connection.

I suggest we check if we are on Slime and do not quit in that case.
A very simple way is to check the `TERM` environment variable. On Slime, it's "dumb". I made a trivial system for that: [termp](https://github.com/vindarel/termp).