Make Pageant use the same SID-selection logic as the Pageant client

Make Pageant use the same SID-selection logic as the Pageant client
code (as introduced in r9043), so that it uses the user SID rather
than the default SID.

This does change the access-control model, in that a Pageant running
with administrator privilege will now serve keys to an unprivileged
PuTTY running as the same user who started Pageant. Owen and I think
this isn't a problem (in particular, it will still not serve keys to a
_different_ user).

More importantly, making the Pageant client and server code work the
same way means that PuTTY and Pageant can still talk to each other
when UAC is turned off, which we've had several reports of r9043
having broken.

[originally from svn r9178]
[r9043 == 05f22632eb45c9b8d17e8ffe7aa97e2590f8f11c]