Workaround for the SSH2 RSA padding bug in OpenSSH 2.5 - 3.2

inclusive. Padding is accomplished by rewriting the signature blob
rather than at the point of generation, in order to avoid having to
move part of the workaround into Pageant (and having to corrupt the
agent wire protocol to allow PuTTY to specify whether it wants its
signatures padded!).

git-svn-id: svn://svn.tartarus.org/sgt/putty@1708 cda61777-01e9-0310-a592-d414129be87e