Update setup script with better logic for (sanely) resolving symbolic links

Specifically, if the setup script is a symbolic link, these links are
resolved (recursively, even); but if the dotfiles directory itself is a
symbolic link, these links are not resolved.

Examples:

If dotfiles is located at /etc/dotfiles, the directory is symlinked to
~/.dotfiles, and the user sources ~/.dotfiles/setup, the script will
generate relative symbolic links to ~/.dotfiles/etc/*.

If dotfiles is located at /etc/dotfiles, the setup script is symlinked
to ~/.dotfiles, and the user sources ~/.dotfiles, the script will
generate absolute symbolic links to /etc/dotfiles/*.
On branch master
Your branch is up-to-date with 'origin/master'.

Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   setup