Determines whether a character is a control character.
Unlike the standard C library
iscntrl() function, this only
recognizes standard ASCII control characters and ignores the
FALSE for all non-ASCII characters. Also,
unlike the standard library function, this takes a char, not
an int, so don’t call it on
EOF, but no need to cast to #guchar
before passing a possibly non-ASCII character in.
This function is not directly available to language bindings.