use MySQL terminology for "virtual columns"

For MySQL compatibility and simply because it's better MariaDB should change the terminology and code around "virtual columns" as follows:

• use "generated", not "virtual" or "computed" in error messages
• use GENERATED ALWAYS AS in SHOW CREATE TABLE
• use STORED, not PERSISTENT
• show VIRTUAL GENERATED and STORED GENERATED (not PERSISTENT) in SHOW FIELDS
• update the manual accordingly
• rename ER_.... constants accordingly (but keep compatibility aliases)
• this task does not cover changing the code, like renaming methods and variables