Don't concat directories to file names

[Richard Stallman]

    -(defcustom image-dired-dir "~/.emacs.d/image-dired/"
    +(defcustom image-dired-dir (concat user-emacs-directory "image-dired/")

This has the right result, but for clarity such code really ought to
use expand-file-name.

    address@hidden user-emacs-directory
    +This variable holds the name of the @file{.emacs.d} directory.  It is
    +ordinarily @file{~/.emacs.d}, but differs on some platforms.

The text should make it explicit that this is a directory name,
not a file name.