ssh: add capability detection at startup Because older servers don't return any output for unknown commands, it's tricky to add new commands. The approach is this: we add a "hello" command that reports any interesting capabilities (and other things that might be of interest in the future). To detect whether this new command is supported, we issue both it and our startup detection command ("between") at the beginning of a connection.

ssh: gather initial output so we can do capability detection

Initial implementation of hello command for ssh

update test-help output after new options.