New Plink operating mode: 'plink -shareexists'.

A Plink invocation of the form 'plink -shareexists <session>' tests
for a currently live connection-sharing upstream for the session in
question. <session> can be any syntax you'd use with Plink to make the
actual connection (a host/port number, a bare saved session name,
-load, whatever).

I envisage this being useful for things like adaptive proxying - e.g.
if you want to connect to host A which you can't route to directly,
and you might already have a connection to either of hosts B or C
which are viable proxies, then you could write a proxy shell script
which checks whether you already have an upstream for B or C and goes
via whichever one is currently active.

Testing for the upstream's existence has to be done by actually
connecting to its socket, because on Unix the mere existence of a
Unix-domain socket file doesn't guarantee that there's a process
listening to it. So we make a test connection, and then immediately
disconnect; hence, that shows up in the upstream's event log.

putty.h

@@ -445,6 +445,9 @@ struct backend_tag {
      */
     void (*unthrottle) (void *handle, int);
     int (*cfg_info) (void *handle);
+    /* Only implemented in the SSH protocol: check whether a
+     * connection-sharing upstream exists for a given configuration. */
+    int (*test_for_upstream)(const char *host, int port, Conf *conf);
     const char *name;
     int protocol;
     int default_port;