For each character in string, if the character is not in valid_chars,
replaces the character with substitutor. Modifies string in place,
and return string itself, not a copy. The return value is to allow
nesting such as g_ascii_strup (g_strcanon (str, "abc", '?')).
For each character in string, if the character is not in valid_chars, replaces the character with substitutor. Modifies string in place, and return string itself, not a copy. The return value is to allow nesting such as g_ascii_strup (g_strcanon (str, "abc", '?')).