Added caching of database directories that did not have a db.opt file.
This was common for older MariaDB installaiton or if a user created
a database with 'mkdir'.
Other things:
- Give a note "no db.opt file" if one uses SHOW CREATE DATABASE one
a database without a db.opt file.
This patch changes the main name of 3 byte character set from utf8 to
utf8mb3. New old_mode UTF8_IS_UTF8MB3 is added and set TRUE by default,
so that utf8 would mean utf8mb3. If not set, utf8 would mean utf8mb4.
This commit adds a new feature to the server to add comments at the database
level. 1024 bytes is the maximum comment length allowed. If the comment length
exceeds this limit, a new error/warning code 4144 is thrown, based on whether
thd->is_strict_mode() is true/false. The database comment is also added to the
db.opt file, as well as to the information_schema.schemata table.
