Support NO_AUTO_CREATE_USER sql_mode

[morgo]

Feature Request

Is your feature request related to a problem? Please describe:

Currently the sql-mode NO_AUTO_CREATE_USER is parsed but ignored. This mode is important, since the MySQL permission model is otherwise vulnerable to spelling errors causing unathenticated users to have admin privileges. Consider the following hypothetical case:

-- A new user joins an organization, and is given read only access
CREATE USER 'morgantocker' IDENTIFIED BY 'sekret';
GRANT SELECT ON *.* TO 'morgantocker';

-- Time elapses.
-- The user is then given admin privileges by the organization:
GRANT ALL ON *.* TO 'morgantucker';

-- But there was a common spelling error, 'morgantucker' instead of 'morgantocker' !
-- This leads to a really bad situation, a new user is created with `ALL` and no password!

Describe the feature you'd like:

The sql_mode NO_AUTO_CREATE_USER, to be enabled by default (next GA) as it is in MySQL.

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

N/A