* custom.el: Custom themes no longer use load-path.
(custom-theme-load-path): New option. Change built-in theme
directory to etc/.
(custom-enabled-themes): Add custom-theme-load-path dependency.
(custom-theme--load-path): New function.
(load-theme, custom-available-themes): Use it.
* cus-theme.el (describe-theme-1): Use custom-theme--load-path.
(customize-themes): Link to custom-theme-load-path variable.
