Return information about a fontset FONTSET on frame FRAME.
FONTSET is a fontset name string, nil for the fontset of FRAME, or t
for the default fontset. FRAME nil means the selected frame.
The value is a char-table whose elements have this form:
((FONT OPENED-FONT ...) ...)
FONT is a name of font specified for a range of characters.
OPENED-FONT is a name of a font actually opened.
The char-table has one extra slot. If FONTSET is not the default
fontset, the value the extra slot is a char-table containing the
information about the derived fonts from the default fontset. The format is the same as above. (fn FONTSET &optional FRAME)