core-cli: backport more refactors from rewrite

Still a long way to go, but this introduces a few niceties for
debugging CLI failures:

+ The (extended) output of the last bin/doom command is now logged to
  ~/.emacs.d/.local/doom.log
+ If an error occurs, short backtraces are displayed whether or not you
  have debug mode on. The full backtrace is written to
  ~/.emacs.d/.local/doom.error.log.
+ bin/doom now aborts with a warning if:
  - The script itself or its parent directory is a symlink. It's fine if
    ~/.emacs.d is symlinked though.
  - Running bin/doom as root when your DOOMDIR isn't in /root/.
  - If you're sporting Emacs 26.1 (now handled in the elisp side rather
    than the /bin/sh shebang preamble).
+ If a 'doom sync' was aborted prematurely, you'll be warned that Doom
  was left in an inconsistent state and that you must run `doom sync`
  again.

May address #3746

core/cli/autoloads.el

@@ -14,7 +14,7 @@ one wants that.")
     auto-mode-alist
     interpreter-mode-alist
     Info-directory-list)
-  "A list of variables to be cached in `doom-autoload-file'.")
+  "A list of variables to be cached in `doom-autoloads-file'.")
 
 (defvar doom-autoloads-files ()
   "A list of additional files or file globs to scan for autoloads.")
@@ -26,7 +26,7 @@ one wants that.")
 (defun doom-autoloads-reload (&optional file)
   "Regenerates Doom's autoloads and writes them to FILE."
   (unless file
-    (setq file doom-autoload-file))
+    (setq file doom-autoloads-file))
   (print! (start "(Re)generating autoloads file..."))
   (print-group!
    (cl-check-type file string)