* Now we take into account the call hierarchy when use SQL SECURITY
option.
Added new context variable in SYSTEM namespace - EFFICIENT_USER which is
returns user name in which context a code works.
We change efficient user before call procedure and function, fetch a
record from selective procedure and before execute a trigger.
* Renamed new context variable to EFFECTIVE_USER. Fixed nested calls.
* Improved error messages to print effective user when there is no permission.
* Added description of new context variable EFFECTIVE_USER. Improved description of SQL SECURITY clause.
