Start of an SSH-server-specific config structure.

This is much simpler than Conf, because I don't expect to have to copy
it around, load or save it to disk (or the Windows registry), or
serialise it between processes. So it can be a straightforward struct.

As yet there's nothing actually _in_ it. I've just created the
structure and arranged to pass it through to all the SSH layers. But
now it's here, it will be a place I can add configuration items as I
find I need them.

defs.h

@@ -79,6 +79,7 @@ typedef struct MontgomeryPoint MontgomeryPoint;
 typedef struct EdwardsCurve EdwardsCurve;
 typedef struct EdwardsPoint EdwardsPoint;
 
+typedef struct SshServerConfig SshServerConfig;
 typedef struct SftpServer SftpServer;
 typedef struct SftpServerVtable SftpServerVtable;