When a merge table is opened compare column and key definition of
underlying tables against column and key definition of merge table.

If any of underlying tables have different column/key definition
refuse to open merge table.


mysql-test/r/merge.result:
  A test case for WL#3456.
mysql-test/t/merge.test:
  A test case for WL#3456.
sql/ha_myisam.cc:
  Moved a part of code that converts TABLE object to MyISAM key and column
  definition to a separate function.
  
  Added check_definition() function that compares MyISAM table definitions.
sql/ha_myisammrg.cc:
  Check if underlying table definition conforms to merge table definition,
  and in case underlying table definition differs, refuse to open merge
  table.