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.

unix/uxplink.c

@@ -607,6 +607,7 @@ int main(int argc, char **argv)
     int errors;
     int use_subsystem = 0;
     int got_host = FALSE;
+    int just_test_share_exists = FALSE;
     unsigned long now;
     struct winsize size;
 
@@ -685,6 +686,8 @@ int main(int argc, char **argv)
                     --argc;
 		    provide_xrm_string(*++argv);
 		}
+	    } else if (!strcmp(p, "-shareexists")) {
+                just_test_share_exists = TRUE;
 	    } else {
 		fprintf(stderr, "plink: unknown option \"%s\"\n", p);
 		errors = 1;
@@ -959,6 +962,19 @@ int main(int argc, char **argv)
 	!conf_get_str_nthstrkey(conf, CONF_portfwd, 0))
 	conf_set_int(conf, CONF_ssh_simple, TRUE);
 
+    if (just_test_share_exists) {
+        if (!back->test_for_upstream) {
+            fprintf(stderr, "Connection sharing not supported for connection "
+                    "type '%s'\n", back->name);
+            return 1;
+        }
+        if (back->test_for_upstream(conf_get_str(conf, CONF_host),
+                                    conf_get_int(conf, CONF_port), conf))
+            return 0;
+        else
+            return 1;
+    }
+
     /*
      * Start up the connection.
      */

unix/uxpty.c

@@ -1218,6 +1218,7 @@ Backend pty_backend = {
     pty_provide_logctx,
     pty_unthrottle,
     pty_cfg_info,
+    NULL /* test_for_upstream */,
     "pty",
     -1,
     0

unix/uxser.c

@@ -591,6 +591,7 @@ Backend serial_backend = {
     serial_provide_logctx,
     serial_unthrottle,
     serial_cfg_info,
+    NULL /* test_for_upstream */,
     "serial",
     PROT_SERIAL,
     0