utf8.c: Fix warning category and subcategory conflicts

utf8.c: Fix warning category and subcategory conflicts

The warnings categories non_unicode, nonchar, and surrogate are all
subcategories of 'utf8'.  One should never call a packWARN() with both a
category and a subcategory of it, as it will mean that one can't
completely make the subcategory independent.  For example,

    use warnings 'utf8';
    no warnings 'surrogate';

surrogate warnings will be output if they are tested with a

    ckWARN2(WARN_UTF8, WARN_SURROGATE);

utf8.c was guilty of this.