Fixed bug in ALTER TABLE ... MODIFY integer-column
Added ref_or_null optimization (needed for subqueries)


BUILD/compile-pentium-valgrind-max:
  Remove FORCE_INIT_OF_VARS to spot wrong LINT_INIT() options
mysql-test/r/distinct.result:
  Update of test results (new optimizer)
mysql-test/r/null_key.result:
  Update after ref_or_null optimization
mysql-test/r/subselect.result:
  Update after ref_or_null optimization
mysql-test/t/null_key.test:
  New tests for ref_or_null optimization
sql/sql_select.cc:
  Added ref_or_null optimization
  Optimized find_best_combinations() and read-functions
sql/sql_select.h:
  Added ref_or_null optimization
sql/sql_yacc.yy:
  Fixed bug in ALTER TABLE ... MODIFY integer-column
sql/table.cc:
  Safety fix for ALTER TABLE .. MODIFY
sql/unireg.cc:
  Safety fix for ALTER TABLE .. MODIFY