WL#7726: System account auth plugin

There are several use cases for privileged accounts which should prohibit all
connections using them:

* Proxy base user
* Stored programs and functions
* Events

This is best handled with the ability to explicitly restrict new connections for
a specific account, but can be done for 5.5 and 5.6 today using an
authentication plugin which always returns false (no connections allowed).  This
would be a meaningful stop-gap solution until 5.7 GA implements the preferred
solution.

FR1:  Accounts may be defined using an authentication plugin, "mysql_no_login".

FR2:  Accounts defined using "mysql_no_login" authentication plugin will reject
all client connections.

FR3:  Accounts defined using "mysql_no_login" authentication plugin may be
assigned privileges, like any user.

FR4:  Accounts defined using "mysql_no_login" authentication plugin may be used
as the DEFINER or INVOKER for stored programs, events and views.

FR5:  Accounts defined using "mysql_no_login" authentication plugin may be used
as a base user for proxy users.

I-1:  A new authentication plugin, mysql_no_login, will be provided with MySQL
Server packages.  This plugin will require manual installation (not enabled by
default), and will prevent user accounts configured to use it from accepting
client connections.  It will allow privileges to be granted to associated
accounts for privilege abstraction in stored programs and views.  There will be
no new status variables or configuration options added.