load_math_font(), available_math_fonts() and check_math_fonts() came
before load_font() and available_fonts(), which replace them. They keep
working as they did: load_font() also loads a math font, and
available_fonts() lists the math fonts (math is TRUE).
Value
load_math_font() returns NULL, invisibly. available_math_fonts()
returns a character vector of math font names. check_math_fonts()
prints the math fonts that are loaded and whether the bundled font files
are present, and returns their names, invisibly.