r6437 broke the case where Pageant is running but contains no SSH-2 keys

r6437 broke the case where Pageant is running but contains no SSH-2 keys
that the SSH-2 server is happy with. Fixed, and since I'm here, fix
`pubkeyfile-and-pageant' as well (for SSH-1 and SSH-2).
Also, in SSH-2, we now reexamine "methods that can continue" for every
Pageant key offer, which is technically more correct although it seems
unlikely that it was causing any real problems.
(It's not entirely pretty, but neither was the old code. We could probably
do with some sort of abstraction for public/private keys to avoid carting
lots of fiddly bits of data around.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@6459 cda61777-01e9-0310-a592-d414129be87e