Remove some unneeded messages, don't panic on TCP socket errors

Fixes hanging at the end of "git push"

This allows it to work when use for eg git

Requires alloc. Only tested with std.

Some other RAM reductions

We don't need OwnedSig state any more (we send straight away), simplify
packet creation.

sunsetc host -s sftp

would fail before. argh has a "greedy" attribute to handle that

Saves around 5kB in picow demo, functions can be shared between
EncodeBytes/EncodeLen/EncodeHash

Could eventually perhaps go behind a rsa feature

Instead of exiting

github.com disconnects immediately. OpenSSH and Dropbear clients
don't send it anyway.

sunsetc can now push itself to github!

Not perfect, it won't handle
-a option

etc after the hostname

Is now sufficient to run "sftp -S sunsetc"

The Behaviour makes a separate request for pty or shell etc once the
channel is established. This gets rid of InitReqs which wasted space.

Subsystem is also supported (not tested yet)

Doesn't do anything, but response with Failure rather than Unimplemented

The async trait requires nightly to build

Do away with the lifetime failures by removing the 'm lifetime
altogether.

This adds a mess of <C: CliBehaviour, S: ServBehaviour>
all over the place. It's an experiment, perhaps it can be tidied.

sshclient/CmdlineClient has lifetime problems to fix.

Not currently used or tested, pending figuring how to do
an async agent_sign() in CliBehaviour.

Our own type doesn't implement std::error::Error,
which makes it easier to add a From<std::error:Error> impl
to simplify the error paths.

This is simpler and avoids needing to add an 'alloc' feature
to Salty

Split things in preparation of giving picow hardware-specific menus

The note in rfc4253 seems informational rather than a requirement.
OpenSSH sends chacha20poly1305 packets shorter than that (such as
newkeys).