server: Add $tls shell variable for captive nbdkit

Author: rwmjones

docs/nbdkit-captive.pod

@@ -67,6 +67,12 @@ guestfish and expanded differently.  Since NBD URIs are now widely
 supported this magic is no longer necessary.  In nbdkit E<ge> 1.24
 both variables expand to the same URI.
 
+=item C<$tls>
+
+Corresponds to the I<--tls> option passed to nbdkit.  If I<--tls=off>
+this is not set.  If I<--tls=on> this is set to C<"1">.  If
+I<--tls=require> this is set to C<"2">.
+
 =item C<$port>
 
 If E<ne> "", the port number that nbdkit is listening on.

server/captive.c

@@ -118,7 +118,9 @@ run_command (void)
   shell_quote (export_name, fp);
   putc ('\n', fp);
 
-  /* Construct $port and $unixsocket. */
+  /* Construct $tls, $port and $unixsocket. */
+  if (tls > 0)
+    fprintf (fp, "tls=%d\n", tls);
   fprintf (fp, "port=");
   if (port)
     shell_quote (port, fp);