sshpeer: extract the forward output logic We are about to make a more aggressive use of this when reading and writing on the other pipes. We it needs to be reusable.

sshpeer: break "OutOfBandError" feature for ssh (BC) When we'll be using the ssh's 'stderr' for realtime output successfully, it will no longer be possible to use 'stderr' to carry the error message (because it is already consumed by the real time output logic. This feature is very rarely used (test says largefile only) and I think breaking its output pattern is worth the benefit of having real time output with ssh.

error: allow a 'hint' to OutOfBandError This will be useful when changing the behavior of OutOfBandError for ssh in the next changeset.