MDEV-7286 TRIGGER: CREATE OR REPLACE, CREATE IF NOT EXISTS

Based on the patch by Sriram Patil, made under terms of GSoC 2014.

mysql-test/t/create.test

@@ -1622,12 +1622,18 @@ END ; |
 INSERT INTO t1 (pk, int_key) SELECT `pk` , `int_key` FROM B ;
 
 --delimiter |
---error ER_NOT_SUPPORTED_YET
+--error ER_TRG_ALREADY_EXISTS
 CREATE TRIGGER f BEFORE INSERT ON t1 FOR EACH ROW 
 BEGIN 
   UPDATE A SET `pk`=1 WHERE `pk`=0 ; 
 END ;|
 
+--error ER_NOT_SUPPORTED_YET
+CREATE TRIGGER f1 BEFORE INSERT ON t1 FOR EACH ROW 
+BEGIN 
+  UPDATE A SET `pk`=1 WHERE `pk`=0 ; 
+END ;|
+
 --delimiter ;
 
 DROP TABLE t1;