suggested in review by Heikki.
innobase/include/data0type.h:
Improve comment of dtype_struct.
innobase/include/data0type.ic:
Rename innobase_get_mb_cset() to innobase_get_cset_width().
dtype_get_fixed_size(): Add an extra integrity check for
type->prtype, type->mbminlen and type->mbmaxlen.
sql/ha_innodb.cc:
Rename innobase_get_mb_cset() to innobase_get_cset_width()
