"alter table from MyISAM to MERGE lost data without errors and warnings"
  Add new handlerton flag which prevent user from altering table storage
  engine to storage engines which would lose data. Both 'blackhole' and 
  'merge' are marked with the new flag.
  Tests included.


mysql-test/r/blackhole.result:
  test for bug#10952
mysql-test/r/merge.result:
  test for bug#10952
mysql-test/t/blackhole.test:
  test for bug#10952
mysql-test/t/merge.test:
  test for bug#10952
sql/ha_blackhole.cc:
  Bug#10952
    shouldn't be able to alter a table into a blackhole
sql/ha_myisammrg.cc:
  Bug#10952
    shouldn't be able to alter a table into a merge
sql/handler.h:
  Bug#10952
    new handlerton flag
sql/sql_table.cc:
  Bug#10952
    If alter is changing engine, check if new engine allows creating table
    via ALTER statement.