mailcap.el: Avoid xdg-open silent failure

* lisp/net/mailcap.el (mailcap-view-file): Use 'pipe :connection-type
instead of 'pty to prevent killing of background process on handler
exit.  Avoid regression similar to Bug#44824.

Problem happens only in some desktop environments where mailcap handler
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 launch 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 as well, so the change
does not affect "needsterminal" and "copiousoutput" mailcap features,
they are not supported as earlier.

If main process of the handler fails then show a message with exit
reason.  Output (including error messages) is ignored as before.
Gtk applications tend to report significant amount of failed asserts
hardly informative for majority of users (bug#12972).

Copyright-paperwork-exempt: yes

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