* rename valid_utf8 variable to check_for_valid_utf8
* reorder testing (c == 127) and check_for_valid_utf8 so that
  any non UTF-8 bytes found then make subsequent bytes >= 127
  go thro the one test and not two before jumping down to
  the 'suspecious-ness' test ie. it's an optimization.
* rename not_utf8_continuation_byte to remove the "not_" and
   then use C's "!" operator to say not!
* unindent check_*_byte_utf8_char labels for consistency