Adding a multibyte-aware VARCHAR copying function, to put correct column prefix,
  taking in account number of characters (instead just limiting on number of bytes).
  For example, for a KEY(col(3)) on a UTF8 column when copying the string 'foo bar foo',
  we should put only 3 leftmost characters: 'foo'.
  9 characters were incorrectly put before this fix.


mysql-test/r/ctype_utf8.result:
  Adding test case
mysql-test/t/ctype_utf8.test:
  Adding test case
sql/field_conv.cc:
  Adding multibyte aware copy function for VARCHAR