bug#62991: 30.0.50; normal-backup-enable-predicate does not consider /dev/shm

[Vasilij Schneidermann <mail@vasilij.de>]

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I've found that the `pass` password-store creates temporary files under
/dev/shm rather than /tmp and editing those files with emacs creates
backup files (which may leak passwords outside that directory if
`backup-directory-alist` is customized to place them elsewhere)`. This
seems to be due to `normal-backup-enable-predicate` specifically testing
for /tmp (and two other temporary directories), but not other
directories backed by a tmpfs file system (such as /dev/shm).  There
does not appear to be a customizable to extend the list of directories
it's checking for either. One more candidate for exclusion is the
/run/user/<uid> directory (it's backed by tmpfs as well, but has
different uses).


In GNU Emacs 30.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
 3.24.37, cairo version 1.17.8) of 2023-04-15 built on odonien
Repository revision: c60b59e04c3776a90adaa8c8fe53af3075a833b8
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
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