Remove spurious * in custom docstrings.

Remove spurious * in custom docstrings.
(dframe-xemacsp): Remove, use (featurep 'xemacs) instead.
(dframe-xemacs20p): Remove, inline at the sole use point.
(defface): Don't defvar the face, don't use old-style backquote.
(defcustom): Don't use old-style backquote.
(dframe-frame-parameter, dframe-mouse-event-p):
Make it obvious that it's always defined.
(dframe-popup-kludge): New function to replace
dframe-xemacs-popup-kludge and dframe-xemacs-popup-kludge.
(dframe-frame-mode, dframe-set-timer-internal)
(dframe-mouse-set-point): Remove use of with-no-warnings from
XEmacs-specific code.
(dframe-set-timer-internal): Fix very old bug with
post-command-idle-hook.