into parts when converting to Unicode.
m_ctype.h:
  Reorganizing mb_wc return codes to be able
  to return "an unassigned N-byte-long character".
sql_string.cc:
  Adding code to detect and properly handle
  unassigned characters (i.e. the those character
  which are correctly formed according to the 
  character specifications, but don't have Unicode
  mapping).
Many files:
  Fixing conversion function to return new codes.
ctype_ujis.test, ctype_gbk.test, ctype_big5.test:
  Adding a test case.
ctype_ujis.result, ctype_gbk.result, ctype_big5.result:
  Fixing results accordingly.