The main problem was already fixed by Igor under terms of 16674.
Adding some additional minor fixes and tests.


include/m_ctype.h:
  Adding reference to CHARSET_INFO.txt
mysql-test/r/ctype_utf8.result:
  Adding test case
mysql-test/t/ctype_utf8.test:
  Adding test case
strings/CHARSET_INFO.txt:
  Adding comment about max_sort_char
strings/ctype-mb.c:
  Restiring that non-Unicode character sets use 0xFF as pad character
  for max_str. Only Unicode character sets use wc_mb.
strings/ctype-utf8.c:
  Fixed that max_sort_char for UTF8 from U+00FF to U+FFFF.