WL#8048: Non-intrusive refactoring of Refactoring of Security_context class code

This WL is a step in refactoring of Security_context class code. It collects
the source code changes which do not affect behavior (non-intrusive changes).
These are like making a public class member and provide public getter/setter to it.

NF1: Code refactoring should not effect the current behavior.

NF2: Should work on all platforms.

Most of the Security_context class members are public. Code accessing and
modifying them is spread in many files of MySQL code. As part of this work
log, all public data members of Security_context are converted to private
ones and public accessor methods are introduced for them.

THD::security_ctx is also converted to private data member of THD and
public accessor methods are introduced. 

The code refactoring will be done in the following phases,

Phase 1:
--------
New getter method for "THD::security_ctx: member will be introduced. Places
where "THD::security_ctx" is accessed directly will be changed to use the
getter function "THD::security_context()".

Phase 2:
--------
All the public data members of Security_context class will be converted to
private members of it and accessor methods are introduced for them.

Following data members are converted to private members of Security_context
class,
  * Security_context::user
  * Security_context::priv_user
  * Security_context::proxy_user
  * Security_context::priv_host
  * Security_context::host_or_ip
  * Security_context::master_access
  * Security_context::db_access
  * Security_context::password_expired.

Getter methods for the char array type data members will return object of
type LEX_CSTRING. So that caller can just access value but not modify.

Code accessing Security_context::members directly will be replaced to use
the getter method. And code modifying these members will be replaced with
setter method.