Added ALTER ONLINE TABLE, which will give an error if the change can't be done 'instantly' (without a table copy)

mysql-test/r/alter_table_online.result:
  Test new feature
mysql-test/t/alter_table_online.test:
  Test new feature
sql/handler.cc:
  Added comment
sql/lex.h:
  Added ONLINE keyword
sql/mysql_priv.h:
  Added option to alter table to require online operation
sql/share/errmsg.txt:
  Added error message if ONLINE can't be done
sql/sql_lex.h:
  Added online option
sql/sql_parse.cc:
  Added online option to mysql_alter_table()
sql/sql_table.cc:
  Added test that gives error if table can't be done instantly when requsted to be online.
  Fixed wrong test if table includes a VARCHAR
  Fixed wrong (but unlikely) handling of error conditions in ALTER table
sql/sql_yacc.yy:
  Added ALTER ONLINE TABLE syntax
storage/maria/ha_maria.cc:
  Fixed bug where 'start_bulk_insert' used too small buffer if used with unknown number of rows

sql/handler.cc

@@ -3532,6 +3532,9 @@ handler::ha_delete_table(const char *name)
   Drop table in the engine: public interface.
 
   @sa handler::drop_table()
+
+  The difference between this and delete_table() is that the table is open in
+  drop_table().
 */
 
 void