WL#8006: Remove the deprecated old_password plugin

Remove the deprecated mysql_old_passwords plugin (and the accompanying client
plugin, secure-auth etc).


User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-5.html

Changes to the reference manual occur throughout. Here are some
of the locations:

http://dev.mysql.com/doc/refman/5.7/en/old-native-authentication-plugin.html
http://dev.mysql.com/doc/refman/5.7/en/server-system-
variables.html#sysvar_secure_auth
http://dev.mysql.com/doc/refman/5.7/en/server-system-
variables.html#sysvar_old_passwords
http://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_old-
password
http://dev.mysql.com/doc/refman/5.7/en/password-hashing.html
http://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html

FRQ1: --secure-auth is accepted but a no-op. To be removed in 5.8
FRQ2: --skip-secure-auth is an error
FRQ3: mysql_old_password server plugin is removed
FRQ4: mysql_old_password libmysql plugin is removed
FRQ5: accounts with mysql_old_password plugin are not activated when starting
the server and an "unknown plugin" error is printed in the server log.
FRQ6: OLD_PASSWORD() function is removed in 5.7.
FRQ7: setting old_passwords=1 causes an error. 0 and 2 are still acceptable values.
FRQ8: OLD_PASSWORD() is marked as deprecated in 5.6.

From http://rb.no.oracle.com/rb/r/6019/

WL#8006: remove mysql_old_password

  * Marked server-side --secure-auth as a "zombie" option: can't set it to
anything but 1. Throws a warning when used.
  * Marked client-side --secure-auth option as a "zombie" option. Returns a
deprecation warning when used.
  * Removed mysql_old_passwords server plugin
  * Removed mysql_old_passwords libmysql plugin
  * old_passwords server side option does not allow setting to 1. So it's only 0
and 2
  * Removed the OLD_PASSWORD() SQL function.
  * Multiple internal special cases clean up from the server.
  * Cleanup of the test cases testing old_password
  * Update of the relevant test cases to cover the new functionality.

Added bug #19285177 to track the push of FRQ8 to 5.6