(facemenu-unlisted-faces): Improve doc strings

(facemenu-unlisted-faces): Improve doc strings
of t and nil values.
(facemenu-set-face): Handle START and END interactively.
(facemenu-set-foreground): Don't use a face; specify color directly.
(facemenu-set-background): Likewise.
(facemenu-set-face-from-menu): Doc fix.
(facemenu-active-faces): Use face-attribute-vector
to handle bare attributes not in faces.
(facemenu-get-face): Don't handle face names fg:... and bg:... specially.
(facemenu-add-new-face): New argument MENU.
New way to handle adding colors to the color menus.