WL#2990: SHOW GRANTS: Extend to include SESSION_USER privileges

MySQL will extend the SHOW GRANTS statement to support 
the return of privileges for the SESSION_USER. 
 
Rationale 
--------- 
Currently, one can execute 
SHOW GRANTS FOR CURRENT_USER; 
to get the privileges for the current authenticated user. 
One should also be able to execute 
SHOW GRANTS FOR SESSION_USER; 
to get the privileges for the user who has initiated an 
SQL session. 
 
Currently, there is no real difference between the result of  
these two functions. But CURRENT_USER can switch contexts: 
in cases where an SQL SECURITY DEFINER view/stored routine/trigger 
is executed, the value of CURRENT_USER changes to the ID of the 
definer. Since it's also necessary to be able to identify the user 
whose actions cause the view/stored routine/trigger to activate, 
the value of SESSION_USER may be changed to always return the ID 
of that user. In this case, supporting SHOW GRANTS FOR SESSION_USER 
becomes a valuable extension. 
 
See also: 
WL#3001 Consistent handling of CURRENT_USER in DEFINER execution context 
 
 

The task
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Extend the SHOW GRANTS statement for SESSION_USER.

Syntax
------
SHOW GRANTS FOR SESSION_USER

- returns the GRANT statements that must be issued to duplicate 
the privileges granted to the SESSION_USER user account.