INCOMPATIBLE CHANGE to the SSH2 private key file format. There is

now a passphrase-keyed MAC covering _all_ important data in the
file, including the public blob and the key comment. Should
conclusively scupper any attacks based on nobbling the key file in
an attempt to sucker the machine that decrypts it. MACing the
comment field also protects against a key-substitution attack (if
someone's worked out a way past our DSA protections and can extract
the private key from a signature, swapping key files and
substituting comments might just enable them to get the signature
they need to do this. Paranoid, but might as well).

git-svn-id: svn://svn.tartarus.org/sgt/putty@1413 cda61777-01e9-0310-a592-d414129be87e