update xah-next/previous-emacs-buffer

[emiltoacs]

use xah-user-buffer-p instead of buffer starting with *

Problem: dired eww and help modes are neither user nor emacs buffer.

Solution: change the definition of emacs-buffer to be the logical complement of the user-buffer using xah-user-buffer-p to discriminate between user and emacs buffer

[xahlee]

thank you.