When creating temporary table for UNION, pass TMP_TABLE_FORCE_MYISAM flag to 
create_tmp_table if we will be using fulltext function(s) when reading from the 
temp. table.


mysql-test/r/fulltext_order_by.result:
  Testcase for BUG#11869 (cont'd, eliminating "table type doesn't support FULLTEXT" error)
mysql-test/t/fulltext_order_by.test:
  Testcase for BUG#11869 (cont'd, eliminating "table type doesn't support FULLTEXT" error)
sql/mysql_priv.h:
  BUG#11869 (cont'd, eliminating "table type doesn't support FULLTEXT" error):
  Added TMP_TABLE_FORCE_MYISAM flag
sql/sql_select.cc:
  BUG#11869 (cont'd, eliminating "table type doesn't support FULLTEXT" error):
  In create_tmp_table(), honor the TMP_TABLE_FORCE_MYISAM flag