Fix error message for bad characters in syntax files.

Before this commit, on systems where "char" is a signed type,
formatting a char with %o would sign-extend to the width of "int",
so that a typical error message would look like:
   Bad character in input: `\37777777605'.

With this commit, the value gets zero-extended, producing the more
sensible error mesage:
   Bad character in input: `\205'.