Readjust Pageant's SID check _again_, to make it the union of the

Readjust Pageant's SID check _again_, to make it the union of the
policies before and after r9178, and hence able to talk to both
0.60-like and 0.61-like clients.

I had failed to consider that many pieces of code derived from PuTTY
would have imported the Pageant client code, so we shouldn't randomly
stop supporting things just because _we_ aren't using them any more.

[originally from svn r9264]
[r9178 == af78191a9c8a76281fe35b5e168b794d8cf328f9]