InnoDB. This fixes also a second part of the same problem with prefix keys
on a multibyte string column for InnoDB.


innobase/btr/btr0btr.c:
  Multibyte character set prefix indexes are not any more fixed size. Therefore,
  we have to chect that length of the index field in not greater than
  prefix length.
innobase/rem/rem0cmp.c:
  Remove unnecessary changes.
innobase/row/row0ins.c:
  Fixed unique prefix key or prefix key using multibyte character set bugs for
  InnoDB (BUG #4521). For prefix keys we have to get the storage length
  for the prefix length of characters in the key.
innobase/row/row0row.c:
  Fixed unique prefix key or prefix key using multibyte character set bugs for
  InnoDB (BUG #4521). For prefix keys we have to get the storage length
  for the prefix length of characters in the key.
innobase/row/row0sel.c:
  Fixed unique prefix key or prefix key using multibyte character set bugs for
  InnoDB (BUG #4521). For prefix keys we have to get the storage length
  for the prefix length of characters in the key.
innobase/row/row0upd.c:
  Fixed unique prefix key or prefix key using multibyte character set bugs for
  InnoDB (BUG #4521). For prefix keys we have to get the storage length
  for the prefix length of characters in the key.
mysql-test/r/ctype_utf8.result:
  Added utf8 character test cases for InnoDB.
mysql-test/t/ctype_utf8.test:
  Added utf8 character expected test results for InnoDB.
sql/ha_innodb.cc:
  Added function innobase_get_at_most_n_mbchars to return position of
  the nth character in the multibyte character string.
sql/ha_innodb.h:
  Remove unnecessary changes.