Add exec/subsystem versions of SSH proxying.

This is a simple tweak to the existing in-process SSH jump host
support, where instead of opening a direct-tcpip channel to the
destination host, we open a session channel and run a process in it to
make the connection to the destination.

So, where the existing jump host support replaced a local proxy
command along the lines of "plink %proxyhost -nc %host %port", this
one replaces "plink %proxyhost run-some-command".

Also added a corresponding option to use a subsystem to make the
connection. (Someone could configure an SSH server to support specific
subsystem names for particular destinations, or a general schema of
subsystem names that include the destination address in some standard
format.)

To avoid overflowing the already-full Proxy config panel with an extra
subtype selector, I've put these in as additional top-level proxy
types, so that instead of just PROXY_SSH we now have three
PROXY_SSH_foo.

proxy/proxy.c

@@ -499,7 +499,9 @@ Socket *new_connection(SockAddr *addr, const char *hostname,
         char *proxy_canonical_name;
         Socket *sret;
 
-        if (type == PROXY_SSH &&
+        if ((type == PROXY_SSH_TCPIP ||
+             type == PROXY_SSH_EXEC ||
+             type == PROXY_SSH_SUBSYSTEM) &&
             (sret = sshproxy_new_connection(addr, hostname, port, privport,
                                             oobinline, nodelay, keepalive,
                                             plug, conf, itr)) != NULL)