MDEV-16288 ALTER TABLE…ALGORITHM=DEFAULT does not override alter_algorithm

- ALTER_ALGORITHM should be substituted when there is no mention of
algorithm in alter statement.
- Introduced algorithm(thd) in Alter_info. It returns the
user requested algorithm. If user doesn't specify algorithm explicitly then
it returns alter_algorithm variable.
- changed algorithm() to get_algorithm(thd) to return algorithm name for
displaying the error.
- set_requested_algorithm(algo_value) to avoid direct assignment on
requested_algorithm variable.
- Avoid direct access of requested_algorithm to encapsulate
requested_algorithm variable

mysql-test/suite/innodb/t/alter_algorithm.test

@@ -55,6 +55,10 @@ ALTER TABLE t1 ROW_FORMAT=COMPRESSED;
 --error $error_code
 ALTER TABLE t1 ENGINE=INNODB;
 
+# Irrespective of alter_algorithm value, the following command
+# should succeed because of explicitly mentioning ALGORTHM=DEFAULT
+ALTER TABLE t1 FORCE, ALGORITHM=DEFAULT;
+
 DROP TABLE t1;
 --disable_info