New abstraction 'ConnectionLayer'.

This is a vtable that wraps up all the functionality required from the
SSH connection layer by associated modules like port forwarding and
connection sharing. This extra layer of indirection adds nothing
useful right now, but when I later separate the SSH-1 and SSH-2
connection layer implementations, it will be convenient for each one
to be able to implement this vtable in terms of its own internal data
structures.

To simplify this vtable, I've moved a lot of the logging duties
relating to connection sharing out of ssh.c into sshshare.c: now it
handles nearly all the logging itself relating to setting up
connection sharing in the first place and downstreams connecting and
disconnecting. The only exception is the 'Reusing a shared connection'
announcement in the console window, which is now done in ssh.c by
detecting downstream status immediately after setup.

defs.h

@@ -62,6 +62,7 @@ typedef struct share_channel share_channel;
 
 typedef struct PortFwdManager PortFwdManager;
 typedef struct PortFwdRecord PortFwdRecord;
+typedef struct ConnectionLayer ConnectionLayer;
 
 typedef struct dlgparam dlgparam;