WL#6020: Add Security Chapter to Reference Manual

This task is to satisfy the request that there be better highlighting of security 
features and issues in the MySQL Reference Manual, and that this be visible at a 
high level (e.g., in the table of contents).

Suggestions from Edwin about current sections that should go in such a chapter:

http://dev.mysql.com/doc/refman/5.5/en/user-account-management.html
http://dev.mysql.com/doc/refman/5.5/en/pluggable-authentication.html
http://dev.mysql.com/doc/refman/5.5/en/windows-authentication-plugin.html
http://dev.mysql.com/doc/refman/5.5/en/pam-authentication-plugin.html

This needs some work because it includes only some of the security-related topics, 
and does not address client-side security.

An initial implementation might provide an overview of the relevant topics, with 
cross references to where additional details can be found. We could revise this 
eventually to make the chapter more of a standalone discussion.

Regarding "standalone" status, the current Arby security excerpt should be 
reworked to correspond to what we end up with as a security chapter.

Resulting documentation:

Chapter in Reference Manual:
http://dev.mysql.com/doc/refman/5.5/en/security.html

Topic Guide:
http://dev.mysql.com/doc/mysql-security-excerpt/5.5/en/index.html