Incomplete ALTER TABLE breaks MERGE compatibility
  Fix implicit NOT NULL not set on ALTER of PK columns


mysql-test/r/alter_table.result:
  Test for Bug#6236
mysql-test/t/alter_table.test:
  Test for Bug#6236
sql/sql_table.cc:
  Implicit NOT NULL not set on ALTER of PK columns
BitKeeper/etc/logging_ok:
  Logging to logging@openlogging.org accepted