MDEV-33658 1/2 Refactoring: extract Key length initialization

mysql_prepare_create_table: Extract a Key initialization part that
relates to length calculation and long unique index designation.

append_system_key_parts call also moves there.

Move this initialization before the duplicate elimination.

Extract WITHOUT OVERPLAPS check into a separate function. It had to be moved
earlier in the code to preserve the order of the error checks, as in the tests.

mysql-test/suite/binlog/t/binlog_spurious_ddl_errors.test

@@ -75,7 +75,7 @@ ALTER TABLE t_stmt ADD COLUMN b INT;
 
 CREATE TRIGGER trig_stmt BEFORE INSERT ON t_stmt FOR EACH ROW INSERT INTO t_stmt VALUES (1);
 
---error ER_TOO_MANY_KEY_PARTS
+--error ER_TOO_LONG_KEY
 CREATE INDEX i ON t_stmt(a);
 
 --error ER_BINLOG_ROW_MODE_AND_STMT_ENGINE