Move mainchan into its own file, like agentf.

This gets another big pile of logic out of ssh2connection and puts it
somewhere more central. Now the only thing left in ssh2connection is
the formatting and parsing of the various channel requests; the logic
deciding which ones to issue and what to do about them is devolved to
the Channel implementation, as it properly should be.

defs.h

@@ -62,6 +62,7 @@ typedef struct Ssh Ssh;
 
 typedef struct Channel Channel;
 typedef struct SshChannel SshChannel;
+typedef struct mainchan mainchan;
 
 typedef struct ssh_sharing_state ssh_sharing_state;
 typedef struct ssh_sharing_connstate ssh_sharing_connstate;