New application: 'psusan', the PROT_SSHCONN server end.

In the previous commit I introduced the ability for PuTTY to talk to a
server speaking the bare ssh-connection protocol, and listed several
applications for that ability. But none of those applications is any
use without a server that speaks the same protocol. Until now, the
only such server has been the Unix-domain socket presented by an
upstream connection-sharing PuTTY - and we already had a way to
connect to that.

So here's the missing piece: by reusing code that already existed for
the testing SSH server Uppity, I've created a program that will speak
the bare ssh-connection protocol on its standard I/O channels. If you
want to get a shell session over any of the transports I mentioned in
the last commit, this is the program you need to run at the far end of
it.

I have yet to write the documentation, but just in case I forget, the
name stands for 'Pseudo Ssh for Untappable, Separately Authenticated
Networks'.

sshserver.h

@@ -16,6 +16,7 @@ struct SshServerConfig {
 
     unsigned long ssh1_cipher_mask;
     bool ssh1_allow_compression;
+    bool bare_connection;
 };
 
 Plug *ssh_server_plug(