libsubprocess: set channel flag only when needed

Problem: libsubprocess sets the SUBPROCESS_REXEC_CHANNEL flag
for a remote subprocess if the on_channel_out callback is registered,
but some users (like job-exec) may register a callback and not
request any channels, and then a subprocess server implementation
that doesn't support channels will balk.

Only set that flag when subprocess channels have been defined.

Author: garlick

src/common/libsubprocess/remote.c

@@ -601,7 +601,7 @@ int remote_exec (flux_subprocess_t *p)
     flux_future_t *f;
     int flags = 0;
 
-    if (p->ops.on_channel_out)
+    if (zlist_size (cmd_channel_list (p->cmd)) > 0)
         flags |= SUBPROCESS_REXEC_CHANNEL;
     if (p->ops.on_stdout)
         flags |= SUBPROCESS_REXEC_STDOUT;