Expose structure tags for the connection-sharing data types.

This was a particularly confusing piece of type-danger, because three
different types were passed outside sshshare.c as 'void *' and only
human vigilance prevented one coming back as the wrong one. Now they
all keep their opaque structure tags when they move through other
parts of the code.

defs.h

@@ -55,6 +55,10 @@ typedef struct ssh_tag *Ssh;
 
 typedef struct Channel Channel;
 
+typedef struct ssh_sharing_state ssh_sharing_state;
+typedef struct ssh_sharing_connstate ssh_sharing_connstate;
+typedef struct share_channel share_channel;
+
 /* Note indirection: for historical reasons (it used to be closer to
  * the OS socket type), the type that most code uses for a socket is
  * 'Socket', not 'Socket *'. So an implementation of Socket or Plug