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/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