MDEV-10421 duplicate CHECK CONSTRAINTs.

mysql_prepare_create_table fixed so it doesn't let duplicating
        constraint names. Syntax for CONSTRAINT IF NOT EXISTS added
        and handled in mysql_alter_table.

include/mysql.h

@@ -137,6 +137,7 @@ typedef unsigned long long my_ulonglong;
 /* backward compatibility define - to be removed eventually */
 #define ER_WARN_DATA_TRUNCATED WARN_DATA_TRUNCATED
 #define WARN_PLUGIN_DELETE_BUILTIN ER_PLUGIN_DELETE_BUILTIN
+#define ER_FK_DUP_NAME ER_DUP_CONSTRAINT_NAME
 
 typedef struct st_mysql_rows {
   struct st_mysql_rows *next;		/* list of rows */