Support expressions (functions) for field defaults (table definition), ALTER, INSERT, CREATE?

MariaDB 10.2 supports expressions for field defaults. However pMA escapes and quotes the expression, causing it to become an ordinary string.

```sql
ALTER TABLE `photo_likes` CHANGE `created` `created` INT(11) NOT NULL DEFAULT unix_timestamp();
```

https://mariadb.com/kb/en/library/create-table/#default