Low-level API to open nonstandard port forwardings.

The new portfwdmgr_connect_socket() works basically like the existing
portfwdmgr_connect(), in that it opens an SSH forwarding channel and
gateways it to a Socket. But where portfwdmgr_connect() started from a
(hostname,port) pair and used mgr->conf to inform name lookup and
proxy settings, portfwdmgr_connect_socket() simply takes a callback
that it will call when it wants you to make a Socket for a given Plug,
and that callback can make any kind of Socket it likes.

The idea is that this way you can make port forwardings that talk to
things other than genuine TCP connections, by simply providing an
appropriate callback.

My immediate use case for this is for agent forwarding, and will
appear in the next commit. But it's easy to imagine other purposes you
might use a thing like this for, such as forwarding SSH channels to
AF_UNIX sockets in general.

ssh.h

@@ -380,6 +380,9 @@ void portfwdmgr_close_all(PortFwdManager *mgr);
 char *portfwdmgr_connect(PortFwdManager *mgr, Channel **chan_ret,
                          char *hostname, int port, SshChannel *c,
                          int addressfamily);
+char *portfwdmgr_connect_socket(PortFwdManager *mgr, Channel **chan_ret,
+                                Socket *(*connect)(void *, Plug *), void *ctx,
+                                SshChannel *c);
 bool portfwdmgr_listen(PortFwdManager *mgr, const char *host, int port,
                        const char *keyhost, int keyport, Conf *conf);
 bool portfwdmgr_unlisten(PortFwdManager *mgr, const char *host, int port);