This patch:

1. Introduces a new system variable default_collation_for_utf8mb4 with two
   possible values: utf8_general_ci and utf8mb4_0900_ai_ci.
   The value of the new variable is used as a default implicit collation
   values if the character set is utf8mb4 in:

   * SET NAMES utf8mb4

   * _utf8mb4 '...', _utf8mb4 x... and _utf8mb4 b... literals

   * CREATE/ALTER SCHEMA/DATABASE ... CHARACTER SET utf8mb4

   * CREATE/ALTER TABLE ... DEFAULT CHARACTER SET utf8mb4

   * ALTER TABLE ... /CONVERT TO CHARACTER SET utf8mb4

   * table column definitions in CREATE/ALTER TABLE

   * SP/SF/trigger/event parameter, local variable and return value declarations

   * SHOW COLLATION and SHOW CHARACTER SET output

2. Removes undocumented use cases of the DEFAULT keyword in:

   * CREATE/ALTER TABLE/DATABASE ... [DEFAULT] COLLATE DEFAULT

   * CREATE/ALTER TABLE/DATABASE ... [DEFAULT] CHARACTER SET DEFAULT

   * ALTER TABLE ... CONVERT TO CHARACTER SET DEFAULT

   * ALTER TABLE ... CONVERT TO ... COLLATE DEFAULT

   * LOAD DATA ... CHARACTER SET DEFAULT

   * CREATE FUNCTION(<name> <type> COLLATE DEFAULT)

   * CREATE PROCEDURE(<name> <type> COLLATE DEFAULT)

   * DECLARE ... COLLATE DEFAULT

   * CREATE FUNCTION ... RETURNS ... COLLATE DEFAULT

   * SET NAMES ... COLLATE DEFAULT

   * SET NAMES DEFAULT COLLATE ...