SSH2: don't count data length against window size

it is my hope that the issues 8c9e7a890e5755ee27fca66590ff5d2fa0616e56 and ccb1c3e2bdefd41cb3a7a21876eb0f5cd055c601 sought to resolve were fixed 9f8d8a7bf6c92d49f71dcadf585c898fcccd5c0d and 7a2c7a414c08d28f0700c7f6f8686a9e0e246a44.

Unfortunately, I don't think unit tests are really going to be feasible unless someone knows how to control how OpenSSH decides what window size to use (I don't).