New protocol: PROT_SSHCONN, bare ssh-connection.

This is the same protocol that PuTTY's connection sharing has been
using for years, to communicate between the downstream and upstream
PuTTYs. I'm now promoting it to be a first-class member of the
protocols list: if you have a server for it, you can select it in the
GUI or on the command line, and write out a saved session that
specifies it.

This would be completely insecure if you used it as an ordinary
network protocol, of course. Not only is it non-cryptographic and wide
open to eavesdropping and hijacking, but it's not even _authenticated_
- it begins after the userauth phase of SSH. So there isn't even the
mild security theatre of entering an easy-to-eavesdrop password, as
there is with, say, Telnet.

However, that's not what I want to use it for. My aim is to use it for
various specialist and niche purposes, all of which involve speaking
it over an 8-bit-clean data channel that is already set up, secured
and authenticated by other methods. There are lots of examples of such
channels:

 - a userv(1) invocation
 - the console of a UML kernel
 - the stdio channels into other kinds of container, such as Docker
 - the 'adb shell' channel (although it seems quite hard to run a
   custom binary at the far end of that)
 - a pair of pipes between PuTTY and a Cygwin helper process
 - and so on.

So this protocol is intended as a convenient way to get a client at
one end of any those to run a shell session at the other end. Unlike
other approaches, it will give you all the SSH-flavoured amenities
you're already used to, like forwarding your SSH agent into the
container, or forwarding selected network ports in or out of it, or
letting it open a window on your X server, or doing SCP/SFTP style
file transfer.

Of course another way to get all those amenities would be to run an
ordinary SSH server over the same channel - but this approach avoids
having to manage a phony password or authentication key, or taking up
your CPU time with pointless crypto.

putty.h

@@ -365,7 +365,7 @@ enum {
 
 enum {
     /* Protocol back ends. (CONF_protocol) */
-    PROT_RAW, PROT_TELNET, PROT_RLOGIN, PROT_SSH,
+    PROT_RAW, PROT_TELNET, PROT_RLOGIN, PROT_SSH, PROT_SSHCONN,
     /* PROT_SERIAL is supported on a subset of platforms, but it doesn't
      * hurt to define it globally. */
     PROT_SERIAL,
@@ -1746,6 +1746,7 @@ extern const struct BackendVtable telnet_backend;
  * Exports from ssh.c.
  */
 extern const struct BackendVtable ssh_backend;
+extern const struct BackendVtable sshconn_backend;
 
 /*
  * Exports from ldisc.c.