Add some config directives to map small cap fonts

So in the bad old days of Postscript you had to create separate small caps fonts because thse PS1 format was too simple to handle this kind of complexity.

Opentype now has built-in facilities to manage small caps within the same font.

However, font authors got used to the separate small caps font thing, and still release the small caps part as separate opentype fonts.

Please add facilities in fontconfig to state "this font family is the small caps block of this other font family, present them all as other font family, and map small caps from the separate font family to the target one"