org.el: Avoid xdg-open silent failure

* lisp/org.el (org-open-file): Use 'pipe :connection-type instead of
'pty to prevent killing of background process on handler exit.
(Bug#44824)

Problem happens only in some desktop environments where configured
through `org-file-apps' or mailcap handlers launches actual viewer
(as defined in .desktop files and obtained from mimeapps.list)
in background.  E.g. xdg-open invokes "gio open" or kde-open5 for Gnome
or KDE accordingly and these handlers launches e.g. eog or okular in
background.  As soon as main process exits, temporary terminal session
created by `start-process-shell-command' is terminated.  As a result
background processes receive SIGHUP.

Previously command were executed with no buffer, so the change
does not affect "needsterminal" and "copiousoutput" mailcap features,
they are not supported as earlier.

If handler main process fails then show a message with exit reason.
Output (including error messages) is ignored as before.
Gtk application tends to report significant amount of failed asserts
hardly informative for majority of users.

TINYCHANGE

lisp/org.el

@@ -8734,7 +8734,21 @@ If the file does not exist, throw an error."
 
       (save-window-excursion
 	(message "Running %s...done" cmd)
-	(start-process-shell-command cmd nil cmd)
+        ;; Handlers such as "gio open" and kde-open5 start viewer in background
+        ;; and exit immediately.  Avoid `start-process' since it assumes
+        ;; :connection-type 'pty and kills children processes with SIGHUP
+        ;; when temporary terminal session is finished.
+        (make-process
+         :name "org-open-file" :connection-type 'pipe :noquery t
+         :buffer nil ; use "*Messages*" for debugging
+         :sentinel (lambda (proc event)
+                     (when (and (memq (process-status proc) '(exit signal))
+                                (/= (process-exit-status proc) 0))
+                       (message
+                        "Command %s: %s."
+                        (mapconcat #'identity (process-command proc) " ")
+                        (substring event 0 -1))))
+         :command (list shell-file-name shell-command-switch cmd))
 	(and (boundp 'org-wait) (numberp org-wait) (sit-for org-wait))))
      ((or (stringp cmd)
 	  (eq cmd 'emacs))