Introduce a new checkbox and command-line option to inhibit use of

Introduce a new checkbox and command-line option to inhibit use of
Pageant for local authentication. (This is a `don't use Pageant for
authentication at session startup' button rather than a `pretend
Pageant doesn't exist' button: that is, agent forwarding is
independent of this option.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@6572 cda61777-01e9-0310-a592-d414129be87e