Do not make a file description blocking while it is still in use

use_stdio_socket means that stdin and stdout FDs use the same file
description, which is certainly the case for socket-based services.
Before this change, it was set to false in this case.  This would cause
the file description to be made blocking while it is still in use,
potentially causing deadlocks.

git blame points to commit 0802df7
("Factor out process_child_io"), but I suspect the actual bug dates back
as far as the introduction of socket-based services in
76697e3 ("Working socket-based qrexec").

Fixes: QubesOS/qubes-issues#9169

Author: DemiMarie

libqrexec/process_io.c

@@ -116,7 +116,7 @@ int process_io(const struct process_io_request *req) {
     pid_t local_status = -1;
     pid_t remote_status = -1;
     int stdout_msg_type = is_service ? MSG_DATA_STDOUT : MSG_DATA_STDIN;
-    bool use_stdio_socket = false;
+    bool use_stdio_socket = stdin_fd == stdout_fd;
 
     int ret;
     struct pollfd fds[FD_NUM];