Replace enum+union of local channel types with a vtable.

There's now an interface called 'Channel', which handles the local
side of an SSH connection-layer channel, in terms of knowing where to
send incoming channel data to, whether to close the channel, etc.

Channel and the previous 'struct ssh_channel' mutually refer. The
latter contains all the SSH-specific parts, and as much of the common
logic as possible: in particular, Channel doesn't have to know
anything about SSH packet formats, or which SSH protocol version is in
use, or deal with all the fiddly stuff about window sizes - with the
exception that x11fwd.c's implementation of it does have to be able to
ask for a small fixed initial window size for the bodgy system that
distinguishes upstream from downstream X forwardings.

I've taken the opportunity to move the code implementing the detailed
behaviour of agent forwarding out of ssh.c, now that all of it is on
the far side of a uniform interface. (This also means that if I later
implement agent forwarding directly to a Unix socket as an
alternative, it'll be a matter of changing just the one call to
agentf_new() that makes the Channel to plug into a forwarding.)

unix/uxpgnt.c

@@ -161,13 +161,17 @@ int sshfwd_write(struct ssh_channel *c, const void *data, int len)
 void sshfwd_write_eof(struct ssh_channel *c) { }
 void sshfwd_unclean_close(struct ssh_channel *c, const char *err) { }
 void sshfwd_unthrottle(struct ssh_channel *c, int bufsize) {}
+void sshfwd_window_override_removed(struct ssh_channel *c) { }
+void chan_remotely_opened_confirmation(Channel *chan) { }
+void chan_remotely_opened_failure(Channel *chan, const char *err) { }
+int chan_no_eager_close(Channel *chan, int s, int r) { return FALSE; }
+
 Conf *sshfwd_get_conf(struct ssh_channel *c) { return NULL; }
 void sshfwd_x11_sharing_handover(struct ssh_channel *c,
                                  void *share_cs, void *share_chan,
                                  const char *peer_addr, int peer_port,
                                  int endian, int protomajor, int protominor,
                                  const void *initial_data, int initial_len) {}
-void sshfwd_x11_is_local(struct ssh_channel *c) {}
 
 /*
  * These functions are part of the plug for our connection to the X